Solve y = -x² - 8x - 20: Finding Where Function Values Are Negative

Question

Look at the following function:

y=x28x20 y=-x^2-8x-20

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

f(x) < 0

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify the discriminant to determine the nature of the roots.
  • Step 2: Analyze the quadratic function for signs across its domain.

Now, let's work through each step:

Step 1: Calculate the discriminant D=b24ac D = b^2 - 4ac .
For the quadratic y=x28x20 y = -x^2 - 8x - 20 , we have:

  • a=1 a = -1
  • b=8 b = -8
  • c=20 c = -20

The discriminant is D=(8)24(1)(20)=6480=16 D = (-8)^2 - 4(-1)(-20) = 64 - 80 = -16 . Because D<0 D < 0 , the quadratic has no real roots, meaning it never crosses the x-axis.

Step 2: Since the discriminant is negative and the coefficient a a is negative, the parabola opens downward.
With no x-intercepts, the vertex is the point where the maximum value occurs. However, for a downward opening parabola with D<0 D < 0 , it resides above the x-axis for all x x .

We conclude that the entire parabola remains below the x-axis across all real x x , meaning the function is negative for all x x .

Therefore, the solution to the problem is that the function is negative for all x x .

Answer

The function is negative for all x x .