Solve (x-1/2)(-x+7/2) < 0: Finding Negative Function Values

Quadratic Inequalities with Sign Analysis

Look at the following function:

y=(x12)(x+312) y=\left(x-\frac{1}{2}\right)\left(-x+3\frac{1}{2}\right)

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

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

❤️ Continue Your Math Journey!

We have hundreds of course questions with personalized recommendations + Account 100% premium

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Look at the following function:

y=(x12)(x+312) y=\left(x-\frac{1}{2}\right)\left(-x+3\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

To solve this problem, we'll begin by finding the roots of the quadratic equation y=(x12)(x+312) y = \left(x - \frac{1}{2}\right)\left(-x + 3\frac{1}{2}\right) .

First, set each factor equal to zero:

  • x12=0 x - \frac{1}{2} = 0 gives x=12 x = \frac{1}{2}
  • x+312=0-x + 3\frac{1}{2} = 0 gives x=312 x = 3\frac{1}{2}

This means the roots of the quadratic are x=12 x = \frac{1}{2} and x=312 x = 3\frac{1}{2} .

Next, analyze the intervals determined by these roots:

  • Interval 1: x<12 x < \frac{1}{2}
  • Interval 2: 12<x<312 \frac{1}{2} < x < 3\frac{1}{2}
  • Interval 3: x>312 x > 3\frac{1}{2}

Perform a sign test within these intervals:

  • For x<12 x < \frac{1}{2} : Both x12 x - \frac{1}{2} and x+312 -x + 3\frac{1}{2} are negative, thus their product is positive (negative times negative is positive).
  • For 12<x<312 \frac{1}{2} < x < 3\frac{1}{2} : The factor x12 x - \frac{1}{2} is positive and x+312-x + 3\frac{1}{2} is positive as well, thus product is positive (positive times positive is positive).
  • For x>312 x > 3\frac{1}{2} : x12 x - \frac{1}{2} is positive, but x+312-x + 3\frac{1}{2} is negative, so the product is negative (positive times negative is negative).

Therefore, the quadratic function is negative for: x>312 x > 3\frac{1}{2} and x<12 x < \frac{1}{2} .

The solution to the problem is: x>312 x > 3\frac{1}{2} or x<12 x < \frac{1}{2} .

3

Final Answer

x>312 x > 3\frac{1}{2} or x<12 x < \frac{1}{2}

Key Points to Remember

Essential concepts to master this topic
  • Factored Form: Use zeros to identify critical points and intervals
  • Sign Testing: Check one value per interval: x=0 gives (+)(+)=positive
  • Verification: Test boundary values: at x=2, both factors equal zero ✓

Common Mistakes

Avoid these frequent errors
  • Incorrect sign analysis between roots
    Don't assume the function is negative between the roots = wrong solution! For this problem, the function is actually positive between 1/2 and 7/2. Always test a point in each interval to determine the actual sign.

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 zeros first?

+

The zeros divide the number line into intervals where the function doesn't change sign. Finding zeros at x=12 x = \frac{1}{2} and x=312 x = 3\frac{1}{2} gives you the critical points!

How do I know which intervals are negative?

+

Pick any test point in each interval and substitute it into the original function. If the result is negative, that entire interval satisfies f(x)<0 f(x) < 0 .

What if I get the opposite answer?

+

Double-check your sign analysis! Remember: negative × negative = positive and positive × negative = negative. Also verify that you correctly converted 312=72 3\frac{1}{2} = \frac{7}{2} .

Why is the answer 'or' instead of 'and'?

+

The function is negative in two separate regions: x<12 x < \frac{1}{2} OR x>312 x > 3\frac{1}{2} . Use 'and' only when x must satisfy both conditions simultaneously.

Do I include the zeros in my answer?

+

No! Since we want f(x)<0 f(x) < 0 (strictly less than), the zeros where f(x)=0 f(x) = 0 are not included in the solution.

🌟 Unlock Your Math Potential

Get unlimited access to all 18 The Quadratic Function questions, detailed video solutions, and personalized progress tracking.

📹

Unlimited Video Solutions

Step-by-step explanations for every problem

📊

Progress Analytics

Track your mastery across all topics

🚫

Ad-Free Learning

Focus on math without distractions

No credit card required • Cancel anytime

More Questions

Click on any question to see the complete solution with step-by-step explanations