Examples with solutions for Product Representation: Finding Increasing or Decreasing Domains

Exercise #1

Find the intervals of increase and decrease of the function:

y=(2x12)(x214) y=\left(2x-\frac{1}{2}\right)\left(x-2\frac{1}{4}\right)

Video Solution

Step-by-Step Solution

To solve the problem of finding intervals of increase and decrease for the given function, follow these steps:

  • Step 1: Expand the function.
    We start by expanding y=(2x12)(x214) y = \left(2x - \frac{1}{2}\right)\left(x - 2\frac{1}{4}\right) :
    y=2xx2x21412x+12214 y = 2x \cdot x - 2x \cdot 2\frac{1}{4} - \frac{1}{2} \cdot x + \frac{1}{2} \cdot 2\frac{1}{4} .
    Simplifying, we get:
    y=2x292x+1294 y = 2x^2 - \frac{9}{2}x + \frac{1}{2} \cdot \frac{9}{4} .
    Thus, y=2x292x+98 y = 2x^2 - \frac{9}{2}x + \frac{9}{8} .

  • Step 2: Differentiate the function.
    Differentiate y=2x292x+98 y = 2x^2 - \frac{9}{2}x + \frac{9}{8} with respect to x x :
    dydx=4x92 \frac{dy}{dx} = 4x - \frac{9}{2} .

  • Step 3: Find the critical points.
    Set the first derivative equal to zero:
    4x92=0 4x - \frac{9}{2} = 0 .
    Solving for x x , we get 4x=92 4x = \frac{9}{2} , hence x=98 x = \frac{9}{8} .

  • Step 4: Use the first derivative test.
    Evaluate the sign of dydx \frac{dy}{dx} around the critical point x=98 x = \frac{9}{8} :
    - For x<98 x < \frac{9}{8} , choose x=1 x = 1 : dydx=4(1)92=8292=12 \frac{dy}{dx} = 4(1) - \frac{9}{2} = \frac{8}{2} - \frac{9}{2} = -\frac{1}{2} (negative).
    - For x>98 x > \frac{9}{8} , choose x=2 x = 2 : dydx=4(2)92=8192=72 \frac{dy}{dx} = 4(2) - \frac{9}{2} = \frac{8}{1} - \frac{9}{2} = \frac{7}{2} (positive).
    Thus, the function decreases when x<98 x < \frac{9}{8} and increases when x>98 x > \frac{9}{8} .

Conclusively, the intervals of increase and decrease are:

:x<114,:x>114 \searrow: x < 1\frac{1}{4}, \nearrow: x > 1\frac{1}{4} .

Answer

:x<114:x>114 \searrow:x<1\frac{1}{4}\\\nearrow:x>1\frac{1}{4}

Exercise #2

Find the intervals of increase and decrease of the function:

y=(3x+1)(4x2) y=\left(3x+1\right)\left(4x-2\right)

Video Solution

Step-by-Step Solution

To solve this problem, follow these steps:

  • Step 1: Expand the function
    We have y=(3x+1)(4x2) y = (3x + 1)(4x - 2) . Expanding this, we get:

y=3x4x+3x(2)+14x+1(2) y = 3x \cdot 4x + 3x \cdot (-2) + 1 \cdot 4x + 1 \cdot (-2)

y=12x26x+4x2 y = 12x^2 - 6x + 4x - 2

y=12x22x2 y = 12x^2 - 2x - 2

  • Step 2: Find the derivative
    The derivative y y' of the function y=12x22x2 y = 12x^2 - 2x - 2 is:

y=ddx(12x22x2) y' = \frac{d}{dx}(12x^2 - 2x - 2)

y=24x2 y' = 24x - 2

  • Step 3: Determine critical points
    Set the derivative equal to zero to find critical points:

24x2=0 24x - 2 = 0

24x=2 24x = 2

x=112 x = \frac{1}{12}

  • Step 4: Determine intervals of increase and decrease
    Evaluate the sign of y=24x2 y' = 24x - 2 on intervals relative to the critical point x=112 x = \frac{1}{12} :

Test values:

- For x<112 x < \frac{1}{12} , choose x=0 x = 0 :

y(0)=24(0)2=2 y'(0) = 24(0) - 2 = -2 (Negative, so decreasing)

- For x>112 x > \frac{1}{12} , choose x=1 x = 1 :

y(1)=24(1)2=22 y'(1) = 24(1) - 2 = 22 (Positive, so increasing)

Therefore, the function is decreasing for x<112 x < \frac{1}{12} and increasing for x>112 x > \frac{1}{12} .

The correct answer is:

:x<112:x>112 \searrow:x<\frac{1}{12}\\\nearrow:x>\frac{1}{12}

Answer

:x<112:x>112 \searrow:x<\frac{1}{12}\\\nearrow:x>\frac{1}{12}

Exercise #3

Find the intervals of increase and decrease of the function

y=(4x+31)2 y=-\left(4x+31\right)^2

Video Solution

Step-by-Step Solution

To determine the intervals of increase and decrease for the function y=(4x+31)2 y = -\left(4x+31\right)^2 , we follow these steps:

  • Step 1: Differentiate the function to find its first derivative.
  • Step 2: Solve for critical points by setting the derivative equal to zero.
  • Step 3: Analyze the sign of the derivative in intervals determined by the critical point.

Now, let's work through each step:

Step 1: Differentiate the function.
The function is y=(4x+31)2 y = -\left(4x+31\right)^2 . Applying the chain rule gives us: y=ddx[(4x+31)2]=2(4x+31)ddx(4x+31)=2(4x+31)4=8(4x+31) y' = \frac{d}{dx}\left[-(4x+31)^2\right] = -2(4x+31) \cdot \frac{d}{dx}(4x+31) = -2(4x+31) \cdot 4 = -8(4x+31) Thus, the derivative is y=8(4x+31) y' = -8(4x+31) .

Step 2: Solve for critical points.
Set the derivative equal to zero: 8(4x+31)=0 -8(4x+31) = 0 4x+31=0 4x+31 = 0 4x=31 4x = -31 x=314=734 x = -\frac{31}{4} = -7\frac{3}{4} This critical point divides the x-axis into two intervals: x<734 x < -7\frac{3}{4} and x>734 x > -7\frac{3}{4} .

Step 3: Analyze the sign of the derivative.
For x<734 x < -7\frac{3}{4} , say x=8 x = -8 : y=8(4(8)+31)=8(32+31)=8(1)=8 y' = -8(4(-8) + 31) = -8(-32 + 31) = -8(-1) = 8 The derivative is positive, indicating the function is increasing.

For x>734 x > -7\frac{3}{4} , say x=0 x = 0 : y=8(0+31)=8×31=248 y' = -8(0 + 31) = -8 \times 31 = -248 The derivative is negative, indicating the function is decreasing.

Thus, the function is increasing for x<734 x < -7\frac{3}{4} and decreasing for x>734 x > -7\frac{3}{4} .

Therefore, the solution to the problem is :x>734:x<734 \searrow:x > -7\frac{3}{4} \\\nearrow:x < -7\frac{3}{4} .

Answer

:x>734:x<734 \searrow:x>-7\frac{3}{4}\\\nearrow:x<-7\frac{3}{4}

Exercise #4

Find the intervals of increase and decrease of the function:

y=(5x1)2 y=\left(5x-1\right)^2

Video Solution

Step-by-Step Solution

To find the intervals of increase and decrease for the function y=(5x1)2 y = (5x - 1)^2 , follow these steps:

  • Step 1: Differentiate the function y=(5x1)2 y = (5x - 1)^2 .
  • Step 2: Set the derivative equal to zero to find critical points.
  • Step 3: Test intervals around the critical point to determine where the function is increasing or decreasing.

Now, let's work through each step.

Step 1: Differentiate the function.
The given function is y=(5x1)2 y = (5x - 1)^2 . Using the chain rule, the derivative y y' is:

y=2(5x1)5=10(5x1)=50x10 y' = 2(5x - 1) \cdot 5 = 10(5x - 1) = 50x - 10

Step 2: Find critical points.
Set y=0 y' = 0 :

50x10=0 50x - 10 = 0

Solving for x x , we get:

50x=10x=15 50x = 10 \quad \Rightarrow \quad x = \frac{1}{5}

Step 3: Determine intervals of increase and decrease.
Test intervals around the critical point x=15 x = \frac{1}{5} .

  • For x<15 x < \frac{1}{5} (e.g., x=0 x = 0 ), y=50(0)10=10 y' = 50(0) - 10 = -10 . Therefore, y<0 y' < 0 , and the function is decreasing.
  • For x>15 x > \frac{1}{5} (e.g., x=1 x = 1 ), y=50(1)10=40 y' = 50(1) - 10 = 40 . Therefore, y>0 y' > 0 , and the function is increasing.

Thus, the function is decreasing on the interval x<15 x < \frac{1}{5} and increasing on the interval x>15 x > \frac{1}{5} .

Therefore, the correct choice is:

:x<15:x>15 \searrow: x < \frac{1}{5} \\ \nearrow: x > \frac{1}{5}

Answer

:x<15:x>15 \searrow:x<\frac{1}{5}\\\nearrow:x>\frac{1}{5}

Exercise #5

Find the intervals of increase and decrease of the function:

y=(13x+17)2 y=\left(\frac{1}{3}x+\frac{1}{7}\right)^2

Video Solution

Step-by-Step Solution

To find where the function y=(13x+17)2 y = \left(\frac{1}{3}x + \frac{1}{7}\right)^2 is increasing or decreasing, follow these steps:

  • Step 1: Calculate the derivative of the function, y=(13x+17)2 y = \left(\frac{1}{3}x + \frac{1}{7}\right)^2 . Use the chain rule: differentiate the outer function and multiply by the derivative of the inner function.
  • Step 2: Differentiate: ddx((13x+17)2)=2(13x+17)×13=23(13x+17) \frac{d}{dx}\left( \left(\frac{1}{3}x + \frac{1}{7}\right)^2 \right) = 2\left(\frac{1}{3}x + \frac{1}{7}\right) \times \frac{1}{3} = \frac{2}{3}\left(\frac{1}{3}x + \frac{1}{7}\right) .
  • Step 3: Set the derivative equal to zero to find critical points: 23(13x+17)=0 \frac{2}{3}\left(\frac{1}{3}x + \frac{1}{7}\right) = 0 .
  • Step 4: Solve for x x : 13x+17=0 \frac{1}{3}x + \frac{1}{7} = 0 implies 13x=17 \frac{1}{3}x = -\frac{1}{7} , leading to x=37 x = -\frac{3}{7} .
  • Step 5: Apply the first derivative test around x=37 x = -\frac{3}{7} : Check sign changes of derivative.
  • Step 6: For x<37 x < -\frac{3}{7} , choose a test point (e.g., x=1 x = -1 ), plug into 23(13x+17) \frac{2}{3}\left(\frac{1}{3}x + \frac{1}{7}\right) and observe negative result indicates decreasing.
  • Step 7: For x>37 x > -\frac{3}{7} , choose another test point (e.g., x=0 x = 0 ), plug in and positive result implies increasing.

Therefore, the function y=(13x+17)2 y = \left(\frac{1}{3}x + \frac{1}{7}\right)^2 decreases for x<37 x < -\frac{3}{7} and increases for x>37 x > -\frac{3}{7} .

Final Solution:

:x<3/7:x>3/7 \searrow:x<-3/7 \\ \nearrow:x>-3/7 , and checking its equivalence in the given answer options as fractions reveals it is choice 1: :x<921:x>921 \searrow:x<\frac{9}{21} \\ \nearrow:x>\frac{9}{21} .

Answer

:x<921:x>921 \searrow:x<\frac{9}{21}\\\nearrow:x>\frac{9}{21}

Exercise #6

Find the intervals of increase and decrease of the function:

y=(x+16)(x419) y=\left(x+\frac{1}{6}\right)\left(-x-4\frac{1}{9}\right)

Video Solution

Step-by-Step Solution

To find the intervals of increase and decrease for the function y=(x+16)(x419) y = \left(x + \frac{1}{6}\right)\left(-x - 4\frac{1}{9}\right) , we follow these steps:

  • Step 1: Expand the function. This means multiplying out the terms:
    y=(x+16)(x379)=x2379x16x3754 y = (x + \frac{1}{6})(-x - \frac{37}{9}) = -x^2 - \frac{37}{9}x - \frac{1}{6}x - \frac{37}{54}
    Simplifying gives y=x222354x3754 y = -x^2 - \frac{223}{54}x - \frac{37}{54} .
  • Step 2: Find the derivative of the function.
    dydx=2x22354 \frac{dy}{dx} = -2x - \frac{223}{54} .
  • Step 3: Find critical points by setting the derivative to zero:
    2x22354=0 -2x - \frac{223}{54} = 0 . Solving for x x gives x=223108 x = -\frac{223}{108} , which simplifies to x=7736 x = -\frac{77}{36} .
  • Step 4: Determine intervals of increase and decrease by testing values around the critical point x=7736 x = -\frac{77}{36} .
    If x<7736 x \lt -\frac{77}{36} , the derivative 2x22354 -2x - \frac{223}{54} is positive, indicating an increasing interval.
    If x>7736 x \gt -\frac{77}{36} , the derivative 2x22354 -2x - \frac{223}{54} is negative, indicating a decreasing interval.

Thus, the function is increasing on the interval x<7736 x \lt -\frac{77}{36} and decreasing on the interval x>7736 x \gt -\frac{77}{36} .

Therefore, the intervals of increase and decrease are:
:x>7736:x<7736 \searrow:x \gt -\frac{77}{36} \\ \nearrow:x \lt -\frac{77}{36}

Answer

:x>7736:x<7736 \searrow:x>-\frac{77}{36}\\\nearrow:x<-\frac{77}{36}

Exercise #7

Find the intervals of increase and decrease of the function:

y=(x34)2 y=\left(x-\frac{3}{4}\right)^2

Video Solution

Step-by-Step Solution

The function given is y=(x34)2 y = \left(x - \frac{3}{4}\right)^2 . This is a quadratic function with its vertex (or minimum point) at x=34 x = \frac{3}{4} .

To find where the function is increasing or decreasing, follow these steps:

  • Step 1: Differentiate y y with respect to x x .
  • y=ddx[(x34)2]=2(x34) y' = \frac{d}{dx}\left[\left(x - \frac{3}{4}\right)^2\right] = 2\left(x - \frac{3}{4}\right)

  • Step 2: Find the critical points by setting the derivative equal to zero.
  • 2(x34)=0x=34 2\left(x - \frac{3}{4}\right) = 0 \quad \Rightarrow \quad x = \frac{3}{4}

  • Step 3: Determine the sign of y y' in the intervals divided by the critical point.
  • For x<34 x < \frac{3}{4} , (x34)<0 \left(x - \frac{3}{4}\right) < 0 and hence y<0 y' < 0 , indicating decreasing behavior.

    For x>34 x > \frac{3}{4} , (x34)>0 \left(x - \frac{3}{4}\right) > 0 and hence y>0 y' > 0 , indicating increasing behavior.

Thus, the function decreases on the interval (,34)(-\infty, \frac{3}{4}), and increases on the interval (34,)(\frac{3}{4}, \infty).

Consequently, the intervals of increase and decrease are:

:x>34:x<34 \searrow: x > \frac{3}{4} \\ \nearrow: x < \frac{3}{4}

Answer

:x>34:x<34 \searrow:x>\frac{3}{4}\\\nearrow:x<\frac{3}{4}

Exercise #8

Find the intervals of increase and decrease of the function:

y=(2x16)2 y=(2x-16)^2

Video Solution

Step-by-Step Solution

To solve this problem, let's follow through the steps:

  • Step 1: Differentiate the function y=(2x16)2 y=(2x-16)^2 with respect to x x .

The derivative of y y is found using the chain rule. The outer function is u2 u^2 with u=(2x16) u = (2x-16) . Thus, the derivative is:

y=2(2x16)ddx(2x16) y' = 2(2x-16) \cdot \frac{d}{dx}(2x-16)

This simplifies to:

y=2(2x16)2=4(2x16)=8x64 y' = 2(2x-16) \cdot 2 = 4(2x-16) = 8x - 64

  • Step 2: Find the critical points by setting the derivative to zero.

Set 8x64=0 8x - 64 = 0 :

8x=64 8x = 64

x=8 x = 8 , so x=8 x = 8 is a critical point.

  • Step 3: Analyze each interval determined by this critical point to determine where y y is increasing or decreasing.

The number line is split into two intervals by the critical point x=8 x = 8 : (,8)(-∞, 8) and (8,) (8, ∞) .

For x<8 x < 8 (e.g., x=7 x = 7 ):

Evaluate y(7)=8(7)64=5664=8 y'(7) = 8(7) - 64 = 56 - 64 = -8 , so y(x)<0 y'(x) < 0 . Thus, y y is decreasing for x<8 x < 8 .

For x>8 x > 8 (e.g., x=9 x = 9 ):

Evaluate y(9)=8(9)64=7264=8 y'(9) = 8(9) - 64 = 72 - 64 = 8 , so y(x)>0 y'(x) > 0 . Thus, y y is increasing for x>8 x > 8 .

Therefore, the function y=(2x16)2 y = (2x-16)^2 is:
Decreasing on the interval (,8) (-∞, 8) and Increasing on the interval (8,) (8, ∞) .

This corresponds to the correct answer choice:

:x<8:x>8 \searrow:x<8\\\nearrow:x>8

Answer

:x<8:x>8 \searrow:x<8\\\nearrow:x>8

Exercise #9

Find the intervals of increase and decrease of the function:

y=(4x+16)2 y=(4x+16)^2

Video Solution

Step-by-Step Solution

To determine where the function y=(4x+16)2 y = (4x + 16)^2 is increasing or decreasing, let's analyze its derivative:

Step 1: Differentiate the function.

The function y=(4x+16)2 y = (4x + 16)^2 is of the form (u(x))2 (u(x))^2 where u(x)=4x+16 u(x) = 4x + 16 . The derivative of y y with respect to x x is:

y=2u(x)u(x) y' = 2u(x) \cdot u'(x)

Here, u(x)=4 u'(x) = 4 , so the derivative is:

y=2(4x+16)4=8(4x+16)=32x+128 y' = 2(4x + 16) \cdot 4 = 8(4x + 16) = 32x + 128 .

Step 2: Find the critical points.

Set the derivative equal to zero and solve for x x :

32x+128=0 32x + 128 = 0

32x=128 32x = -128

x=4 x = -4 .

Step 3: Determine the sign of the derivative around the critical point x=4 x = -4 .

  • Choose a test point less than 4-4, for example x=5 x = -5 . Then, y=32(5)+128=32 y' = 32(-5) + 128 = -32 , which is negative, indicating the function is decreasing.
  • Choose a test point greater than 4-4, for example x=0 x = 0 . Then, y=32(0)+128=128 y' = 32(0) + 128 = 128 , which is positive, indicating the function is increasing.

Therefore, the function is decreasing on the interval x<4 x < -4 and increasing on the interval x>4 x > -4 .

The correct answer choice matches these findings:

:x<4:x>4 \searrow:x<-4\\\nearrow:x>-4

Answer

:x<4:x>4 \searrow:x<-4\\\nearrow:x>-4

Exercise #10

Find the intervals of increase and decrease of the function:

y=(4x+22)2 y=(4x+22)^2

Video Solution

Step-by-Step Solution

To solve this problem, we'll begin by finding the derivative of the given function y=(4x+22)2 y = (4x + 22)^2 with respect to x x .

The function can be expanded as:

y=(4x+22)2=16x2+2422x+222=16x2+176x+484 y = (4x + 22)^2 = 16x^2 + 2 \cdot 4 \cdot 22 \cdot x + 22^2 = 16x^2 + 176x + 484

Next, find dydx \frac{dy}{dx} by differentiating:

dydx=ddx(16x2+176x+484)=32x+176 \frac{dy}{dx} = \frac{d}{dx}(16x^2 + 176x + 484) = 32x + 176

Set the derivative equal to zero to find the critical point:

32x+176=0 32x + 176 = 0

32x=176 32x = -176

x=17632 x = -\frac{176}{32}

x=112 x = -\frac{11}{2}

x=5.5 x = -5.5

This critical point, x=5.5 x = -5.5 , will be the vertex of the parabola, determining where the function changes from decreasing to increasing.

Now, test the sign of dydx=32x+176 \frac{dy}{dx} = 32x + 176 in intervals around the critical point:

  • For x<5.5 x < -5.5 , choose x=6 x = -6 :
  • 32(6)+176=192+176=16 32(-6) + 176 = -192 + 176 = -16

    Since the derivative is negative, the function is decreasing.

  • For x>5.5 x > -5.5 , choose x=5 x = -5 :
  • 32(5)+176=160+176=16 32(-5) + 176 = -160 + 176 = 16

    Since the derivative is positive, the function is increasing.

Therefore, the intervals of increase and decrease are:

:x<512 \searrow:x < -5\frac{1}{2}

:x>512 \nearrow:x > -5\frac{1}{2}

Thus, the correct choice from the given options is:

:x<512 \searrow:x < -5\frac{1}{2}

:x>512 \nearrow:x > -5\frac{1}{2}

Answer

:x<512:x>512 \searrow:x<-5\frac{1}{2}\\\nearrow:x>-5\frac{1}{2}

Exercise #11

Find the intervals of increase and decrease of the function:

y=(4x+32)2 y=-(4x+32)^2

Video Solution

Step-by-Step Solution

To determine the intervals of increase and decrease, follow these steps:

  • Step 1: Simplify the function. The given function is y=(4x+32)2 y = -(4x + 32)^2 , a quadratic in terms of x x .
  • Step 2: Recognize that this is a downward-facing parabola because the coefficient of (4x+32)2 (4x + 32)^2 is negative.
  • Step 3: Find the vertex or the critical point of the parabola. The function is in the form y=a(x+h)2+k y = -a(x + h)^2 + k . Here, a=4 a = 4 , h=8 h = -8 , and k=0 k = 0 .
  • Step 4: The vertex occurs at x=8 x = -8 . The function is symmetrical about this point.
  • Step 5: Since it's a downwards-opening parabola, the function increases on the left of the vertex and decreases on the right of the vertex.
  • Step 6: Thus, the function decreases ( \searrow ) for x<8 x < -8 and increases ( \nearrow ) for x>8 x > -8 .

Consequently, the intervals of increase and decrease are:
Decreasing: x<8 x < -8
Increasing: x>8 x > -8

Therefore, :x<8:x>8 \searrow: x<-8 \\ \nearrow: x>-8 is the correct answer.

Answer

:x<8:x>8 \searrow:x<-8\\\nearrow:x>-8

Exercise #12

Find the intervals of increase and decrease of the function:

y=(5x1)(4x14) y=(5x-1)\left(4x-\frac{1}{4}\right)

Video Solution

Step-by-Step Solution

To find the intervals of increase and decrease for the function y=(5x1)(4x14) y = (5x - 1)\left(4x - \frac{1}{4}\right) , we perform the following steps:

  • Step 1: Differentiate the function using the product rule.
  • Step 2: Find the critical points by setting the derivative to zero.
  • Step 3: Determine the intervals where the derivative is positive or negative to infer increasing and decreasing behavior.

Now, let's work through each step in detail:

Step 1: Differentiate the function.
Using the product rule, consider u=5x1 u = 5x - 1 and v=4x14 v = 4x - \frac{1}{4} . The derivative of the function is:

y=ddx[(5x1)(4x14)]=(5x1)ddx(4x14)+(4x14)ddx(5x1) y' = \frac{d}{dx}\left[(5x - 1)\left(4x - \frac{1}{4}\right)\right] = (5x - 1)\frac{d}{dx}\left(4x - \frac{1}{4}\right) + \left(4x - \frac{1}{4}\right)\frac{d}{dx}(5x - 1)

=(5x1)4+(4x14)5=20x4+20x54 = (5x - 1) \cdot 4 + \left(4x - \frac{1}{4}\right) \cdot 5 = 20x - 4 + 20x - \frac{5}{4}

=40x214 = 40x - \frac{21}{4}

Step 2: Find the critical points.
Set the derivative to zero:

40x214=0 40x - \frac{21}{4} = 0

Solving for x x , multiply both sides by 4 to clear the fraction:

160x21=0 160x - 21 = 0

x=21160 x = \frac{21}{160}

Step 3: Analyze the sign of the derivative around the critical point to determine increasing or decreasing intervals.
Choose a test point in each interval defined by the critical point x=21160 x = \frac{21}{160} .

  • For x<21160 x < \frac{21}{160} , choose x=0 x = 0 and check the sign of y=40(0)214=214 y' = 40(0) - \frac{21}{4} = -\frac{21}{4} which is negative, indicating y y is decreasing.
  • For x>21160 x > \frac{21}{160} , choose x=1 x = 1 and check the sign of y=40(1)214=1394 y' = 40(1) - \frac{21}{4} = \frac{139}{4} which is positive, indicating y y is increasing.

Thus, the function decreases for x<21160 x < \frac{21}{160} and increases for x>21160 x > \frac{21}{160} .

Therefore, the intervals are :x<21160 \searrow:x<\frac{21}{160} and :x>21160 \nearrow:x>\frac{21}{160} .

The correct choice is:

:x<21160:x>21160 \searrow:x<\frac{21}{160}\\\nearrow:x>\frac{21}{160}

Answer

:x<21160:x>21160 \searrow:x<\frac{21}{160}\\\nearrow:x>\frac{21}{160}

Exercise #13

Find the intervals of increase and decrease of the function:

y=(7x+3)(5x2) y=\left(7x+3\right)\left(5x-2\right)

Video Solution

Step-by-Step Solution

To solve this problem, we'll find the intervals of increase and decrease for the function y=(7x+3)(5x2) y = (7x + 3)(5x - 2) .

First, let's expand the function:

y=(7x+3)(5x2)=35x2+15x14x6=35x2+x6 y = (7x + 3)(5x - 2) = 35x^2 + 15x - 14x - 6 = 35x^2 + x - 6 .

Next, compute the derivative y y' :

y=ddx(35x2+x6)=70x+1 y' = \frac{d}{dx}(35x^2 + x - 6) = 70x + 1 .

To find critical points, set y=0 y' = 0 :

70x+1=0 70x + 1 = 0

70x=1 70x = -1

x=170 x = -\frac{1}{70} .

Now, we need to determine the sign of y y' in the intervals around the critical point x=170 x = -\frac{1}{70} :

  • For x<170 x < -\frac{1}{70} , choose a test point such as x=1 x = -1 :
    y=70(1)+1=70+1=69 y' = 70(-1) + 1 = -70 + 1 = -69 , so y<0 y' < 0 , indicating that the function is decreasing in this interval.
  • For x>170 x > -\frac{1}{70} , choose a test point such as x=0 x = 0 :
    y=70(0)+1=1 y' = 70(0) + 1 = 1 , so y>0 y' > 0 , indicating that the function is increasing in this interval.

Putting it all together, we have:

The function is decreasing on the interval x<170 x < -\frac{1}{70} and increasing on the interval x>170 x > -\frac{1}{70} .

Therefore, the intervals of increase and decrease are:

:x<170:x>170 \searrow:x < -\frac{1}{70} \\ \nearrow:x > -\frac{1}{70} .

Answer

:x<170:x>170 \searrow:x<-\frac{1}{70}\\\nearrow:x>-\frac{1}{70}

Exercise #14

Find the intervals of increase and decrease of the function:

y=(13x+12)2 y=\left(\frac{1}{3}x+\frac{1}{2}\right)^2

Video Solution

Step-by-Step Solution

To determine where the function y=(13x+12)2 y=\left(\frac{1}{3}x+\frac{1}{2}\right)^2 is increasing or decreasing, we need to first find its derivative.

Let's compute the derivative y y' of the function:

y=(13x+12)2 y = \left(\frac{1}{3}x + \frac{1}{2}\right)^2 .

Using the chain rule, let u=13x+12 u = \frac{1}{3}x + \frac{1}{2} , then y=u2 y = u^2 .

The derivative of u2 u^2 with respect to x x is 2ududx 2u \cdot \frac{du}{dx} .

Now, find dudx\frac{du}{dx}:

dudx=13 \frac{du}{dx} = \frac{1}{3} .

Thus, the derivative of the function is:

y=2(13x+12)13=23(13x+12) y' = 2 \left( \frac{1}{3}x + \frac{1}{2} \right) \cdot \frac{1}{3} = \frac{2}{3} \left( \frac{1}{3}x + \frac{1}{2} \right) .

Set this derivative to zero to find critical points:

23(13x+12)=0 \frac{2}{3} \left( \frac{1}{3}x + \frac{1}{2} \right) = 0 .

Simplify to find x x :

13x+12=0 \frac{1}{3}x + \frac{1}{2} = 0 .

13x=12 \frac{1}{3}x = -\frac{1}{2} .

x=12×3 x = -\frac{1}{2} \times 3 .

x=32 x = -\frac{3}{2} .

The critical point is at x=32 x = -\frac{3}{2} .

To determine the nature of intervals around this critical point, test y y' on intervals around x=32 x = -\frac{3}{2} .

- For x<32 x < -\frac{3}{2} : Choose x=2 x = -2 .
y=23(13(2)+12)=23(23+12)<0 y' = \frac{2}{3} \left( \frac{1}{3}(-2) + \frac{1}{2} \right) = \frac{2}{3}(-\frac{2}{3} + \frac{1}{2}) < 0 .
y y' is negative, so y y is decreasing.

- For x>32 x > -\frac{3}{2} : Choose x=0 x = 0 .
y=23(13(0)+12)=23×12>0 y' = \frac{2}{3} \left( \frac{1}{3}(0) + \frac{1}{2} \right) = \frac{2}{3} \times \frac{1}{2} > 0 .
y y' is positive, so y y is increasing.

Thus, the function decreases on x<32 x < -\frac{3}{2} and increases on x>32 x > -\frac{3}{2} .

The intervals of increase and decrease are :x>112 \searrow: x > -1\frac{1}{2} and :x<112 \nearrow: x < -1\frac{1}{2} .

Analyzing the multiple-choice answers, the correct one matches choice 3.

Answer

:x>112:x<112 \searrow:x>-1\frac{1}{2}\\\nearrow:x<-1\frac{1}{2}

Exercise #15

Find the intervals of increase and decrease of the function:

y=(2x214)2 y=\left(\right.2x-2\frac{1}{4})^2

Video Solution

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify a simpler form of the equation and its derivative.
  • Step 2: Find critical points where the derivative is zero.
  • Step 3: Determine signs of the derivative on intervals around critical points.

Let's proceed with these steps:

Step 1: Our function is already in vertex form: y=(2x214)2 y = \left(2x - 2\frac{1}{4}\right)^2 . It's important to note that this term can be simplified as y=(2x2.25)2 y = (2x - 2.25)^2 , identifying the vertex at x=1.125 x = 1.125 , which is x=118 x = 1\frac{1}{8} .

Step 2: Differentiate the function with respect to x x . For y=(2x2.25)2 y = (2x - 2.25)^2 , use the chain rule:
y=2(2x2.25)2=4(2x2.25)=8x9 y' = 2(2x - 2.25) \cdot 2 = 4(2x - 2.25) = 8x - 9 .

Step 3: Set the derivative to zero to find critical points:
8x9=0 8x - 9 = 0 leads to x=98=1.125 x = \frac{9}{8} = 1.125 or x=118 x = 1\frac{1}{8} .

The function decreases to the left of this point and increases to the right. Specifically:

  • If x<118 x < 1\frac{1}{8} , 8x9<0 8x - 9 < 0 , so y y is decreasing.
  • If x>118 x > 1\frac{1}{8} , 8x9>0 8x - 9 > 0 , so y y is increasing.

Therefore, the intervals of increase and decrease are:

:x<118 \nearrow:x<1\frac{1}{8} (Increasing: to the left of the vertex),

:x>118 \searrow:x>1\frac{1}{8} (Decreasing: to the right of the vertex).

Answer

:x>118:x<118 \searrow:x>1\frac{1}{8}\\\nearrow:x<1\frac{1}{8}

Exercise #16

Find the intervals of increase and decrease of the function:

y=(x+1)(x+16) y=\left(x+1\right)\left(x+\frac{1}{6}\right)

Video Solution

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Expand the original function
  • Step 2: Differentiate the function
  • Step 3: Find the critical points by setting the derivative to zero
  • Step 4: Determine intervals of increase and decrease based on the derivative's sign

Let's explore each step in detail:
Step 1: Expand the function:

The function y=(x+1)(x+16) y = (x+1)(x+\frac{1}{6}) expands to:

y=x2+16x+x+16=x2+76x+16 y = x^2 + \frac{1}{6}x + x + \frac{1}{6} = x^2 + \frac{7}{6}x + \frac{1}{6} .

Step 2: Differentiate the function:
The derivative of the quadratic function y=x2+76x+16 y = x^2 + \frac{7}{6}x + \frac{1}{6} is:

y=2x+76 y' = 2x + \frac{7}{6} .

Step 3: Set the derivative equal to zero to find critical points:
Solve 2x+76=0 2x + \frac{7}{6} = 0 .

Subtract 76 \frac{7}{6} from both sides: 2x=76 2x = -\frac{7}{6} .

Divide both sides by 2 to solve for x x :

x=712 x = -\frac{7}{12} .

Step 4: Determine the sign of the derivative on either side of the critical point:

- For x<712 x < -\frac{7}{12} , the derivative y=2x+76 y' = 2x + \frac{7}{6} is negative, indicating the function is decreasing.

- For x>712 x > -\frac{7}{12} , the derivative y y' is positive, indicating the function is increasing.

Thus, the function is decreasing on x<712 x < -\frac{7}{12} and increasing on x>712 x > -\frac{7}{12} .

Therefore, the final solution is:

:x<712:x>712 \searrow:x < -\frac{7}{12} \\ \nearrow: x > -\frac{7}{12} .

Answer

:x<712:x>712 \searrow:x<-\frac{7}{12}\\\nearrow:x>-\frac{7}{12}

Exercise #17

Find the intervals of increase and decrease of the function:

y=(x4.4)(x2.3) y=\left(x-4.4\right)\left(x-2.3\right)

Video Solution

Step-by-Step Solution

Let's solve the problem step by step:

  • **Step 1:** Identify the points p p and q q from the function y=(x4.4)(x2.3) y = (x-4.4)(x-2.3) . Here, p=4.4 p = 4.4 and q=2.3 q = 2.3 .
  • **Step 2:** Calculate the vertex x x -coordinate using the formula for the midpoint: x=p+q2=4.4+2.32 x = \frac{p+q}{2} = \frac{4.4 + 2.3}{2} .
  • **Step 3:** Compute the value: x=6.72=3.35 x = \frac{6.7}{2} = 3.35 .
  • **Step 4:** Since the quadratic has a positive leading coefficient after expansion (implying it opens upwards), the function decreases on (,3.35) (-\infty, 3.35) and increases on (3.35,) (3.35, \infty) .

Therefore, the function is decreasing for x<3.35 x < 3.35 and increasing for x>3.35 x > 3.35 .

The correct choice that matches this conclusion is:
:x<3.35:x>3.35 \searrow: x < 3.35 \\\nearrow: x > 3.35

Answer

:x<3.35:x>3.35 \searrow:x<3.35\\\nearrow:x>3.35

Exercise #18

Find the intervals of increase and decrease of the function:

y=(x16)2 y=-(x-16)^2

Video Solution

Step-by-Step Solution

To solve this problem, we need to determine where the function y=(x16)2 y = -(x-16)^2 is increasing and decreasing.

The function given, y=(x16)2 y = -(x-16)^2 , represents a quadratic function with a vertex form of y=a(xh)2+k y = a(x-h)^2 + k , where a=1 a = -1 , h=16 h = 16 , and k=0 k = 0 . This form shows that the parabola opens downwards because the coefficient a=1 a = -1 is negative.

The vertex of the parabola, found at x=16 x = 16 , is the point where the function changes its direction. For x<16 x < 16 , since the parabola opens downwards, the function is increasing as it moves toward the vertex. For x>16 x > 16 , the function is decreasing as it moves away from the vertex.

Thus, the intervals are:
- Increasing: x<16 x < 16
- Decreasing: x>16 x > 16

The correct solution to the problem, which matches the given answer, is: :x>16:x<16 \searrow: x > 16 \\ \nearrow: x < 16 .

Answer

:x>16:x<16 \searrow:x>16\\\nearrow:x<16

Exercise #19

Find the intervals of increase of the function:

y=(x4)(x+2) y=(x-4)(x+2)

Video Solution

Step-by-Step Solution

To solve this problem, we'll determine the intervals of increase by finding the derivative of the quadratic function y=(x4)(x+2) y=(x-4)(x+2) .

  • Step 1: Expand the function.

The function can be written in expanded form as y=x22x8 y = x^2 - 2x - 8 .

  • Step 2: Find the derivative.

The derivative of y=x22x8 y = x^2 - 2x - 8 is y=2x2 y' = 2x - 2 .

  • Step 3: Solve y=0 y' = 0 to find critical points.

Setting the derivative equal to zero gives 2x2=0 2x - 2 = 0 , which simplifies to x=1 x = 1 .

  • Step 4: Determine where y>0 y' > 0 for increasing intervals.

Analyzing the derivative y=2x2 y' = 2x - 2 :

  • If x>1 x > 1 , then y>0 y' > 0 , indicating the function is increasing.
  • If x<1 x < 1 , then y<0 y' < 0 , indicating the function is decreasing.

Therefore, the function y=(x4)(x+2) y = (x-4)(x+2) is increasing for x>1 x > 1 .

As a result, the interval of increase for this function is x>1 x > 1 .

Answer

x>1 x>1

Exercise #20

Find the intervals where the function is decreasing:

y=(x+1)(7x) y=\left(x+1\right)\left(7-x\right)

Video Solution

Step-by-Step Solution

To find the intervals where the function y=(x+1)(7x) y = (x+1)(7-x) is decreasing, we begin by expanding the function:
y=(x+1)(7x)=7xx2+7x=x2+6x+7 y = (x+1)(7-x) = 7x - x^2 + 7 - x = -x^2 + 6x + 7 .

The function is now in the form y=x2+6x+7 y = -x^2 + 6x + 7 .
This is a quadratic function, opening downward because the coefficient of x2 x^2 is negative.

Let's find the critical points by taking the derivative and setting it to zero.
The derivative of y y is y=ddx(x2+6x+7)=2x+6\ y' = \frac{d}{dx}(-x^2 + 6x + 7) = -2x + 6 .
Solving 2x+6=0-2x + 6 = 0 gives x=3 x = 3 .

The vertex, x=3 x = 3 , is where the function changes from increasing to decreasing.

To determine the interval where the function is decreasing, consider the derivative:<br>2x+6<0<br> -2x + 6 < 0.
Solving gives 2x<6 -2x < -6, resulting in x>3 x > 3 .

Therefore, the function is decreasing for x>3 x > 3 .

The correct answer is: x>3 x > 3

Answer

x>3 x>3