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

Quadratic Functions with Negative Discriminant

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 f(x) < 0

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Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

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 f(x) < 0

2

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 .

3

Final Answer

The function is negative for all x x .

Key Points to Remember

Essential concepts to master this topic
  • Rule: Use discriminant D=b24ac D = b^2 - 4ac to determine root behavior
  • Technique: For y=x28x20 y = -x^2 - 8x - 20 , calculate D=6480=16 D = 64 - 80 = -16
  • Check: Since a<0 a < 0 and D<0 D < 0 , function stays below x-axis ✓

Common Mistakes

Avoid these frequent errors
  • Confusing sign analysis when discriminant is negative
    Don't assume a downward parabola with no real roots is positive! This leads to saying the function is never negative. Since the parabola opens downward (a = -1) and never crosses the x-axis (D < 0), it stays entirely below the x-axis. Always consider both the coefficient 'a' and discriminant together.

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

What does it mean when the discriminant is negative?

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A negative discriminant means the quadratic has no real roots - the parabola never touches or crosses the x-axis. It stays entirely on one side of the x-axis.

How do I know if the function is always positive or always negative?

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Look at the coefficient of x2 x^2 ! If a > 0, the parabola opens upward (always positive with no roots). If a < 0, it opens downward (always negative with no roots).

Why can't I just substitute random x-values to check?

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While testing values helps verify, the discriminant method gives you the complete answer instantly! Testing a few points might mislead you about the function's behavior everywhere.

What if I calculated the discriminant wrong?

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Double-check your arithmetic: D=(8)24(1)(20)=6480=16 D = (-8)^2 - 4(-1)(-20) = 64 - 80 = -16 . Remember that two negatives multiply to positive: 4(-1)(-20) = 80.

Could the function ever equal zero?

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No! Since the discriminant is negative, there are no real solutions to x28x20=0 -x^2 - 8x - 20 = 0 . The function never equals zero - it's always negative.

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