Look at the following function:
y=(x−21)(−x+321)
Determine for which values of x the following is true:
f(x) < 0
To solve this problem, we'll begin by finding the roots of the quadratic equation y=(x−21)(−x+321).
First, set each factor equal to zero:
- x−21=0 gives x=21
- −x+321=0 gives x=321
This means the roots of the quadratic are x=21 and x=321.
Next, analyze the intervals determined by these roots:
- Interval 1: x<21
- Interval 2: 21<x<321
- Interval 3: x>321
Perform a sign test within these intervals:
- For x<21: Both x−21 and −x+321 are negative, thus their product is positive (negative times negative is positive).
- For 21<x<321: The factor x−21 is positive and −x+321 is positive as well, thus product is positive (positive times positive is positive).
- For x>321: x−21 is positive, but −x+321 is negative, so the product is negative (positive times negative is negative).
Therefore, the quadratic function is negative for: x>321 and x<21.
The solution to the problem is: x>321 or x<21.
x > 3\frac{1}{2} or x < \frac{1}{2}