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

Question

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

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} .

Answer

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