Solve (x-6)(x+6) > 0: Finding Values Where Function is Positive

Quadratic Inequalities with Sign Analysis

Look at the following function:

y=(x6)(x+6) y=\left(x-6\right)\left(x+6\right)

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

Look at the following function:

y=(x6)(x+6) y=\left(x-6\right)\left(x+6\right)

Determine for which values of x x the following is true:

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

2

Step-by-step solution

To solve this problem, we need to find the values of x x that make (x6)(x+6)>0 (x-6)(x+6) > 0 .

Let's consider the critical points where each factor could change sign by setting each factor equal to zero:

  • x6=0 x-6 = 0 gives x=6 x = 6 .
  • x+6=0 x+6 = 0 gives x=6 x = -6 .

These critical points divide the number line into three intervals:

  • x<6 x < -6
  • 6<x<6 -6 < x < 6
  • x>6 x > 6

We will test each interval to see where f(x)=(x6)(x+6)>0 f(x) = (x-6)(x+6) > 0 :

1. Interval x<6 x < -6 :

If x<6 x < -6 , both (x6) (x-6) and (x+6) (x+6) are negative (e.g., test x=7 x = -7 ).
(x6)(x+6)=()()>0(x-6) \cdot (x+6) = (-)\cdot(-) > 0: The product is positive.

2. Interval 6<x<6 -6 < x < 6 :

If 6<x<6 -6 < x < 6 , (x6) (x-6) is negative, and (x+6) (x+6) is positive (e.g., test x=0 x = 0 ).
(x6)(x+6)=()(+)<0(x-6) \cdot (x+6) = (-)\cdot(+) < 0: The product is negative.

3. Interval x>6 x > 6 :

If x>6 x > 6 , both (x6) (x-6) and (x+6) (x+6) are positive (e.g., test x=7 x = 7 ).
(x6)(x+6)=(+)(+)>0(x-6) \cdot (x+6) = (+)\cdot(+) > 0: The product is positive.

Therefore, the inequality (x6)(x+6)>0 (x-6)(x+6) > 0 holds for x<6 x < -6 or x>6 x > 6 . Thus, the correct answer is:

x>6 x > 6 or x<6 x < -6

3

Final Answer

x>6 x > 6 or x<6 x < -6

Key Points to Remember

Essential concepts to master this topic
  • Critical Points: Set each factor to zero to find boundary values
  • Sign Testing: Test intervals: x = -7 gives (-)(−) = (+) > 0
  • Verification: Check boundary behavior: at x = -6 and x = 6, product equals 0 ✓

Common Mistakes

Avoid these frequent errors
  • Including the boundary points in the solution
    Don't write x6 x ≥ 6 or x6 x ≤ -6 when the inequality is > 0! At x = 6 and x = -6, the product equals zero, not positive. Always use strict inequalities for > or < conditions.

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 we find where each factor equals zero first?

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These are critical points where the function changes from positive to negative (or vice versa). At x=6 x = -6 and x=6 x = 6 , the product equals zero, creating boundaries between different sign regions.

How do I remember which intervals are positive?

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Use the sign chart method! Pick any test value in each interval. For example: test x=7 x = -7 (both factors negative), x=0 x = 0 (one positive, one negative), and x=7 x = 7 (both positive).

What's the difference between > 0 and ≥ 0?

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The symbol > 0 means strictly greater than zero, so we exclude points where the function equals zero. The symbol ≥ 0 would include the boundary points x=6 x = -6 and x=6 x = 6 .

Can I expand the expression first?

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You could expand to get x236>0 x^2 - 36 > 0 , but keeping it factored makes the sign analysis much easier! Factored form shows you exactly where the function changes sign.

Why isn't the answer -6 < x < 6?

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Test a value in that interval! At x=0 x = 0 : (06)(0+6)=(6)(6)=36<0 (0-6)(0+6) = (-6)(6) = -36 < 0 . The product is negative between the critical points, not positive.

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