Solve y = (1/2)x² - 2: Finding Positive Function Values

Quadratic Functions with Sign Analysis

Find the positive and negative domains of the function below:

y=12x22 y=\frac{1}{2}x^2-2

Then determine for which values of x x the following is true:

f(x)>0 f(x) > 0

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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 function below:

y=12x22 y=\frac{1}{2}x^2-2

Then determine for which values of x x the following is true:

f(x)>0 f(x) > 0

2

Step-by-step solution

To solve for the positive and negative domains of the function y=12x22 y = \frac{1}{2}x^2 - 2 , we will go through the following steps:

  • Step 1: Find the roots of the equation by setting the function equal to zero: 12x22=0 \frac{1}{2}x^2 - 2 = 0 .
  • Step 2: Solve 12x2=2 \frac{1}{2}x^2 = 2 , which simplifies to x2=4 x^2 = 4 . Taking the square root of both sides gives x=2 x = 2 or x=2 x = -2 .
  • Step 3: Test intervals between the roots (,2)(- \infty, -2), (2,2)(-2, 2), and (2,) (2, \infty) to determine where the function is positive.

Now, let's determine which intervals yield positive values:

For x<2 x < -2 , pick x=3 x = -3 and plug into the function: y=12(3)22=12(9)2=4.52=2.5 y = \frac{1}{2}(-3)^2 - 2 = \frac{1}{2}(9) - 2 = 4.5 - 2 = 2.5 , which is positive.

For 2<x<2-2 < x < 2, pick x=0 x = 0 : y=12(0)22=2 y = \frac{1}{2}(0)^2 - 2 = -2 , which is negative.

For x>2 x > 2 , pick x=3 x = 3 : y=12(3)22=12(9)2=4.52=2.5 y = \frac{1}{2}(3)^2 - 2 = \frac{1}{2}(9) - 2 = 4.5 - 2 = 2.5 , which is positive.

Thus, the function y=12x22 y = \frac{1}{2}x^2 - 2 is positive for x>2 x > 2 or x<2 x < -2 .

Therefore, the solution is x>2 x > 2 or x<2 x < -2 .

3

Final Answer

x>2 x > 2 or x<2 x < -2

Key Points to Remember

Essential concepts to master this topic
  • Finding Roots: Set function equal to zero and solve for x-intercepts
  • Test Intervals: Pick values between roots like x = -3, 0, 3
  • Sign Check: Substitute test values to confirm where f(x)>0 f(x) > 0

Common Mistakes

Avoid these frequent errors
  • Solving inequality without finding roots first
    Don't try to solve 12x22>0 \frac{1}{2}x^2 - 2 > 0 directly = missing critical boundary points! This skips the essential step of finding where the function changes sign. Always find the roots first by setting the function equal to zero, then test intervals between the roots.

Practice Quiz

Test your knowledge with interactive questions

The graph of the function below intersects the X-axis at points A and B.

The vertex of the parabola is marked at point C.

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

AAABBBCCCX

FAQ

Everything you need to know about this question

Why do I need to find where the function equals zero first?

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Finding the roots (where y=0 y = 0 ) shows you the boundary points where the function changes from positive to negative. These points at x=2 x = -2 and x=2 x = 2 divide the number line into intervals to test!

How do I know which test values to pick?

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Pick any value within each interval! For x<2 x < -2 , try x=3 x = -3 . For 2<x<2 -2 < x < 2 , try x=0 x = 0 . For x>2 x > 2 , try x=3 x = 3 . The exact values don't matter - just make sure they're inside each interval.

Why is the function negative between the roots?

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Because this is a parabola opening upward (coefficient of x2 x^2 is positive). It dips below the x-axis between its roots at x=2 x = -2 and x=2 x = 2 , creating a negative region in the middle.

What does 'or' mean in the final answer?

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The word 'or' means the function is positive in either of these regions: when x<2 x < -2 OR when x>2 x > 2 . These are two separate intervals where f(x)>0 f(x) > 0 is true.

Do I include the boundary points x = -2 and x = 2?

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No! At these points, f(x)=0 f(x) = 0 , not f(x)>0 f(x) > 0 . Since we want strictly positive values, we use strict inequalities like x>2 x > 2 instead of x2 x ≥ 2 .

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