Analyze the Quadratic Function: Domains of y = -2x² + 3x + 2

Quadratic Functions with Sign Analysis

Find the positive and negative domains of the following function:

y=2x2+3x+2 y=-2x^2+3x+2

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Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Find the positive and negative domains of the following function:

y=2x2+3x+2 y=-2x^2+3x+2

2

Step-by-step solution

To solve the given problem, we will perform the following steps:

  • Calculate the roots of the quadratic equation y=2x2+3x+2 y = -2x^2 + 3x + 2 using the quadratic formula: x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} .
  • Given a=2 a = -2 , b=3 b = 3 , and c=2 c = 2 , use the formula:

x=3±324(2)(2)2(2)=3±9+164=3±254 x = \frac{-3 \pm \sqrt{3^2 - 4(-2)(2)}}{2(-2)} = \frac{-3 \pm \sqrt{9 + 16}}{-4} = \frac{-3 \pm \sqrt{25}}{-4}

  • The roots are x=3+54=12 x = \frac{-3 + 5}{-4} = -\frac{1}{2} and x=354=2 x = \frac{-3 - 5}{-4} = 2 .
  • The roots 12-\frac{1}{2} and 22 divide the x-axis into three intervals: (,12) (-\infty, -\frac{1}{2}) , (12,2) (-\frac{1}{2}, 2) , and (2,) (2, \infty) .
  • Test a point from each interval in the function to determine the sign of the function in those intervals:

Choose x=1 x = -1 for interval (,12)(- \infty, -\frac{1}{2}):
Substitute into the function: y=2(1)2+3(1)+2=23+2=3 y = -2(-1)^2 + 3(-1) + 2 = -2 - 3 + 2 = -3 (negative).

Choose x=0 x = 0 for interval (12,2)(- \frac{1}{2}, 2):
Substitute into the function: y=2(0)2+3(0)+2=2 y = -2(0)^2 + 3(0) + 2 = 2 (positive).

Choose x=3 x = 3 for interval (2,)(2, \infty):
Substitute into the function: y=2(3)2+3(3)+2=18+9+2=7 y = -2(3)^2 + 3(3) + 2 = -18 + 9 + 2 = -7 (negative).

Therefore, the positive domain where the function is positive is 12<x<2 -\frac{1}{2} < x < 2 , and the negative domains are x<12 x < -\frac{1}{2} or x>2 x > 2 .

The solution to the problem is:

x>0:12<x<2 x > 0 : -\frac{1}{2} < x < 2

x>2 x > 2 or x<0:x<12 x < 0 : x < -\frac{1}{2}

3

Final Answer

x>0:12<x<2 x > 0 : -\frac{1}{2} < x < 2

x>2 x > 2 or x<0:x<12 x < 0 : x < -\frac{1}{2}

Key Points to Remember

Essential concepts to master this topic
  • Zeros: Find where y=0 y = 0 using quadratic formula
  • Test Points: Check sign in each interval: x=1 x = -1 gives y=3 y = -3 (negative)
  • Verify: Positive domain 12<x<2 -\frac{1}{2} < x < 2 contains test point x=0 x = 0 giving y=2>0 y = 2 > 0

Common Mistakes

Avoid these frequent errors
  • Confusing positive/negative domains with function values
    Don't think positive domain means where x > 0 = wrong intervals! This confuses the input variable with function output. Always find where y > 0 (function is positive) and where y < 0 (function is negative) by testing intervals between zeros.

Practice Quiz

Test your knowledge with interactive questions

The graph of the function below does not intersect the \( x \)-axis.

The parabola's vertex is marked A.

Find all values of \( x \) where
\( f\left(x\right) > 0 \).

AAAX

FAQ

Everything you need to know about this question

What's the difference between positive domain and where x is positive?

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The positive domain means where the function value y>0 y > 0 , not where x>0 x > 0 ! For this parabola, the positive domain is 12<x<2 -\frac{1}{2} < x < 2 , which includes both negative and positive x-values.

Why do I need to find the zeros first?

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The zeros (where y=0 y = 0 ) are the boundary points where the parabola crosses the x-axis. These points divide the x-axis into intervals where the function is either all positive or all negative.

How do I know which intervals to test?

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The zeros x=12 x = -\frac{1}{2} and x=2 x = 2 create three intervals: (,12) (-\infty, -\frac{1}{2}) , (12,2) (-\frac{1}{2}, 2) , and (2,) (2, \infty) . Pick any point from each interval to test.

Why does the parabola change from negative to positive to negative?

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Since the coefficient of x2 x^2 is negative (a=2 a = -2 ), this parabola opens downward. It starts negative, becomes positive between the zeros, then becomes negative again.

Can I just graph this instead of calculating?

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Graphing helps visualize, but you still need to calculate the exact zeros using the quadratic formula to get precise interval boundaries like x=12 x = -\frac{1}{2} and x=2 x = 2 .

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