Determining Negative Regions of the Quadratic: y = -2x² - 8x - 10

Quadratic Functions with Negative Discriminant

Look at the following function:

y=2x28x10 y=-2x^2-8x-10

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=2x28x10 y=-2x^2-8x-10

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: Analyze the quadratic function's graph from given coefficients.
  • Step 2: Calculate discriminant to find root characteristics.
  • Step 3: Discuss the implications on positivity or negativity of f(x) f(x) .

Now, let's work through each step:

Step 1: The quadratic function y=2x28x10 y = -2x^2 - 8x - 10 has a leading coefficient a=2 a = -2 , which is negative. This indicates that the parabola opens downwards, potentially sitting below the x x -axis.

Step 2: Calculate the discriminant Δ=b24ac \Delta = b^2 - 4ac .

Here, b=8 b = -8 , a=2 a = -2 , and c=10 c = -10 . Plug these into the formula:

Δ=(8)24(2)(10)=6480=16 \Delta = (-8)^2 - 4(-2)(-10) = 64 - 80 = -16

Since the discriminant Δ \Delta is negative, there are no real roots. The parabola does not intersect the x x -axis.

Step 3: Discuss implications.

Because the parabola opens downward and has no real roots, it lies entirely below the x x -axis. This means for all real values of x x , f(x)<0 f(x) < 0 .

Therefore, the function y=2x28x10 y = -2x^2 - 8x - 10 is negative for all x x .

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: Negative discriminant means no real roots, parabola doesn't cross x-axis
  • Technique: Calculate Δ=b24ac=6480=16 \Delta = b^2 - 4ac = 64 - 80 = -16
  • Check: Since a = -2 < 0 and no x-intercepts, entire parabola below x-axis ✓

Common Mistakes

Avoid these frequent errors
  • Assuming negative coefficient means function is always negative
    Don't conclude f(x) < 0 just because a = -2 is negative = wrong reasoning! The sign of the leading coefficient only tells you parabola direction (up/down), not its position relative to x-axis. Always check the discriminant to determine if the parabola crosses the x-axis.

Practice Quiz

Test your knowledge with interactive questions

The graph of the function below does not intersect the \( x \)-axis.

The parabola's vertex is marked A.

Find all values of \( x \) where
\( f\left(x\right) > 0 \).

AAAX

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 above or below the x-axis.

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

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Check the leading coefficient! If a > 0 and no real roots, the parabola opens up and stays above x-axis (always positive). If a < 0 and no real roots, it opens down and stays below x-axis (always negative).

Why can't I just plug in a test value like x = 0?

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You could test a value, but using the discriminant method is more reliable and complete. It tells you definitively about the entire function, not just one point.

What if the discriminant were positive instead?

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If Δ>0 \Delta > 0 , the parabola would have two real roots and cross the x-axis twice. Then you'd need to find those roots to determine where f(x) < 0.

Can I verify this by completing the square?

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Absolutely! Completing the square for y=2x28x10 y = -2x^2 - 8x - 10 gives y=2(x+2)22 y = -2(x+2)^2 - 2 . Since the maximum value is -2, the function is indeed always negative.

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