Find the Domain of Increase: y = 3x² - 6x + 4 Quadratic Function

Quadratic Functions with Derivative Analysis

Find the domain of increase of the function:

y=3x26x+4 y=3x^2-6x+4

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find the domains of increase of the function
00:04 We'll use the formula to find the X value at the vertex
00:09 Let's identify the trinomial coefficients
00:14 We'll substitute appropriate values according to the given data, and solve for X
00:20 This is the X value at the vertex point
00:25 The coefficient A is positive, therefore the parabola has a minimum point
00:30 From the graph, we'll deduce the domains of increase of the function
00:36 And this is the solution to the question

Step-by-step written solution

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1

Understand the problem

Find the domain of increase of the function:

y=3x26x+4 y=3x^2-6x+4

2

Step-by-step solution

The function given is y=3x26x+4 y = 3x^2 - 6x + 4 .

First, let's find the derivative of the function, which will help us determine the intervals of increase.

The derivative is given by f(x)=ddx(3x26x+4)=6x6 f'(x) = \frac{d}{dx}(3x^2 - 6x + 4) = 6x - 6 .

Next, find where the derivative is zero to locate critical points. Solve 6x6=0 6x - 6 = 0 to get:

6x=6 6x = 6
x=1 x = 1

The critical point is x=1 x = 1 . This is where the function changes from decreasing to increasing since quadratic functions have one axis of symmetry and a>0 a > 0 : indicating a parabola opening upwards.

To determine the interval of increase, analyze the sign of f(x) f'(x) :

  • For x>1 x > 1 , 6x6>0 6x - 6 > 0 which implies f(x)>0 f'(x) > 0 , so the function is increasing.
  • For x<1 x < 1 , 6x6<0 6x - 6 < 0 which implies f(x)<0 f'(x) < 0 , so the function is decreasing.

Thus, the domain of increase for the function is when x>1 x > 1 .

The correct answer is therefore x>1 x > 1 .

3

Final Answer

x>1 x > 1

Key Points to Remember

Essential concepts to master this topic
  • Rule: Function increases where derivative is positive
  • Technique: Find critical point: set f(x)=6x6=0 f'(x) = 6x - 6 = 0 , so x=1 x = 1
  • Check: Test derivative sign: f(2)=6(2)6=6>0 f'(2) = 6(2) - 6 = 6 > 0 confirms increasing for x>1 x > 1

Common Mistakes

Avoid these frequent errors
  • Confusing critical point with domain of increase
    Don't think x = 1 is the domain of increase = completely wrong answer! The critical point is just where the function changes behavior. Always test the derivative's sign on either side of the critical point to find where it's positive.

Practice Quiz

Test your knowledge with interactive questions

Note that the graph of the function shown below does not intersect the x-axis

The parabola's vertex is A

Identify the interval where the function is decreasing:

XXXAAA

FAQ

Everything you need to know about this question

Why do I need to find the derivative first?

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The derivative tells you the slope at every point! When f(x)>0 f'(x) > 0 , the function is going uphill (increasing). When f(x)<0 f'(x) < 0 , it's going downhill (decreasing).

What's the difference between a critical point and domain of increase?

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A critical point (like x=1 x = 1 ) is where the derivative equals zero. The domain of increase is the entire interval where the function goes up, which is x>1 x > 1 in this case.

How do I know if the parabola opens up or down?

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Look at the coefficient of x2 x^2 ! Since we have 3x2 3x^2 and 3 > 0, the parabola opens upward, so it decreases then increases after the vertex.

Can I solve this without derivatives?

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Yes! For quadratic functions, find the vertex using x=b2a x = -\frac{b}{2a} . Here: x=(6)2(3)=1 x = -\frac{(-6)}{2(3)} = 1 . Since a > 0, the function increases for x>1 x > 1 .

Why test the sign of the derivative?

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Testing the sign tells you the direction of change! Pick any number greater than 1 (like x = 2) and substitute: f(2)=6(2)6=6>0 f'(2) = 6(2) - 6 = 6 > 0 , confirming the function increases there.

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