Solving (x-1/2)(x+6.5): Finding Negative Function Values

Quadratic Inequalities with Sign Analysis

Find the positive and negative domains of the following function:

y=(x12)(x+612) y=\left(x-\frac{1}{2}\right)\left(x+6\frac{1}{2}\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

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1

Understand the problem

Find the positive and negative domains of the following function:

y=(x12)(x+612) y=\left(x-\frac{1}{2}\right)\left(x+6\frac{1}{2}\right)

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

f(x)<0 f(x) < 0

2

Step-by-step solution

Let us solve the problem step by step to find: x x values for which f(x)<0 f(x) < 0 .

Firstly, identify the roots of the function y=(x12)(x+612) y = \left(x - \frac{1}{2}\right)\left(x + 6\frac{1}{2}\right) :

  • The root from (x12)=0 \left(x - \frac{1}{2}\right) = 0 is x=12 x = \frac{1}{2} .
  • The root from (x+612)=0 \left(x + 6\frac{1}{2}\right) = 0 is x=612 x = -6\frac{1}{2} .

These roots divide the real number line into three intervals:

  • x<612 x < -6\frac{1}{2}
  • 612<x<12 -6\frac{1}{2} < x < \frac{1}{2}
  • x>12 x > \frac{1}{2}

To determine where the function is negative, evaluate the sign in each interval:

  • For x<612 x < -6\frac{1}{2} : Both factors (x12) (x - \frac{1}{2}) and (x+612) (x + 6\frac{1}{2}) are negative, so their product is positive.
  • For 612<x<12 -6\frac{1}{2} < x < \frac{1}{2} : (x12) (x - \frac{1}{2}) is negative and (x+612) (x + 6\frac{1}{2}) is positive, thus the product is negative.
  • For x>12 x > \frac{1}{2} : Both factors are positive, so their product is positive.

Hence, the function is negative on the interval: 612<x<12 -6\frac{1}{2} < x < \frac{1}{2} .

3

Final Answer

612<x<12 -6\frac{1}{2} < x < \frac{1}{2}

Key Points to Remember

Essential concepts to master this topic
  • Roots: Set each factor equal to zero to find boundary points
  • Sign Analysis: Test intervals: x=7 x = -7 gives (+)(+) = positive
  • Check: Verify boundaries excluded since f(6.5)=f(0.5)=0 f(-6.5) = f(0.5) = 0

Common Mistakes

Avoid these frequent errors
  • Including boundary points in inequality solution
    Don't write 612x12 -6\frac{1}{2} \leq x \leq \frac{1}{2} for f(x) < 0 = boundary points included! The roots make f(x) = 0, not f(x) < 0. Always use strict inequality symbols when the problem asks for f(x) < 0.

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 the roots first?

+

The roots are where the function equals zero, creating boundaries between positive and negative regions. Without finding x=12 x = \frac{1}{2} and x=612 x = -6\frac{1}{2} , you can't determine where the function changes sign!

How do I know which interval to test?

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Pick any convenient number within each interval. For 612<x<12 -6\frac{1}{2} < x < \frac{1}{2} , try x=0 x = 0 : (012)(0+612)=()(+)=negative (0-\frac{1}{2})(0+6\frac{1}{2}) = (-)(+) = \text{negative}

What if I get confused about positive and negative signs?

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Remember: negative × negative = positive and negative × positive = negative. Draw a number line with your roots marked, then test one point in each section to determine the sign!

Do the boundary points count in the solution?

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Not for strict inequalities! Since we want f(x)<0 f(x) < 0 (not ≤), the boundary points where f(x)=0 f(x) = 0 are excluded from our answer.

Why is the function positive outside the roots?

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This is a quadratic function that opens upward (positive leading coefficient). It's negative between the roots and positive outside the roots - like a U-shape dipping below the x-axis!

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