Solve y=2x²+16: Finding Values Where Function is Negative

Quadratic Functions with Always Positive Values

Given the function:

y=2x2+16 y=2x^2+16

Determine for which values of x the following holds: f(x)<0 f(x) < 0

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Step-by-step written solution

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1

Understand the problem

Given the function:

y=2x2+16 y=2x^2+16

Determine for which values of x the following holds: f(x)<0 f(x) < 0

2

Step-by-step solution

To solve this problem, consider the function y=2x2+16 y = 2x^2 + 16 . Our objective is to find values of x x for which y<0 y < 0 .

The function, y=2x2+16 y = 2x^2 + 16 , is a quadratic function in standard form. Here, a=2 a = 2 , b=0 b = 0 , and c=16 c = 16 .

  • Step 1: Determine the Vertex
    Since the quadratic is in the form y=ax2+bx+c y = ax^2 + bx + c , with a>0 a > 0 , it opens upwards. Therefore, its vertex represents the minimum point. The x-coordinate of the vertex is given by x=b2a=04=0 x = \frac{-b}{2a} = \frac{0}{4} = 0 . Substituting x=0 x = 0 into the function provides y=2(0)2+16=16 y = 2(0)^2 + 16 = 16 .
  • Step 2: Analyze the Discriminant
    The discriminant D=b24ac=024×2×16=128 D = b^2 - 4ac = 0^2 - 4 \times 2 \times 16 = -128 . Since D<0 D < 0 , the quadratic has no real roots, meaning it does not cross the x-axis. Instead, it remains entirely above the x-axis.

Since the discriminant is negative, the function has no real roots, confirming that the quadratic function never takes a value below zero. The vertex is at y=16 y = 16 , which is above zero, indicating all function values are positive.

Therefore, the function y=2x2+16 y = 2x^2 + 16 is never less than zero, implying that there are no values of x x for which y<0 y < 0 .

Hence, the solution is No x.

3

Final Answer

No x

Key Points to Remember

Essential concepts to master this topic
  • Rule: When discriminant is negative, parabola never crosses x-axis
  • Technique: Calculate discriminant: D=024(2)(16)=128 D = 0^2 - 4(2)(16) = -128
  • Check: Find vertex minimum: at x = 0, y = 16 > 0 ✓

Common Mistakes

Avoid these frequent errors
  • Setting the function equal to zero instead of less than zero
    Don't solve 2x² + 16 = 0 when asked for f(x) < 0 = finding roots instead of negative values! This finds where the function crosses zero, not where it's negative. Always analyze the function's behavior relative to the inequality given.

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

Why can't this function ever be negative?

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Because 2x2 2x^2 is always positive or zero (since we're squaring), and adding 16 makes it even larger. The minimum value occurs at x = 0, giving y = 16, which is still positive!

What does the discriminant tell me about negativity?

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When D<0 D < 0 , the parabola doesn't touch the x-axis at all. Since a = 2 > 0, it opens upward and stays entirely above the x-axis, meaning always positive.

How do I know if a quadratic is always positive or negative?

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  • If a > 0 and D < 0: always positive
  • If a < 0 and D < 0: always negative
  • Check the vertex to confirm the minimum/maximum value

Could I graph this to check my answer?

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Absolutely! Graphing y=2x2+16 y = 2x^2 + 16 shows a U-shaped curve with vertex at (0, 16). Since the entire curve sits above the x-axis, there's no point where y < 0.

What if the constant term was negative instead?

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Great question! If we had y=2x216 y = 2x^2 - 16 , then the vertex would be at (0, -16), and the function would be negative between the roots. The positive constant term (+16) keeps this function above zero.

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