Domain Analysis: Finding Valid Inputs for y=-(x-2 6/19)²-2

Quadratic Functions with Vertex Form Analysis

Find the positive and negative domains of the function below:

y=(x2619)22 y=-\left(x-2\frac{6}{19}\right)^2-2

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Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Find the positive and negative domains of the function below:

y=(x2619)22 y=-\left(x-2\frac{6}{19}\right)^2-2

2

Step-by-step solution

To solve this problem, we must analyze the quadratic function y=(x2619)22 y = -\left(x - 2\frac{6}{19}\right)^2 - 2 to determine its positive and negative domains.

  • Step 1: Identify the vertex and direction
    The given function is in the form y=a(xh)2+k y = a(x - h)^2 + k , where a=1 a = -1 , h=2619 h = 2\frac{6}{19} , and k=2 k = -2 . The vertex of the parabola is at (2619,2) (2\frac{6}{19}, -2) .
  • Step 2: Analyze the direction of the parabola
    Since a=1 a = -1 (negative), the parabola opens downward. This indicates the vertex is at the maximum point of the parabola.
  • Step 3: Determine the function's values
    Since the maximum value of the function (at the vertex) is y=2 y = -2 , and the parabola opens downward, the function cannot be positive anywhere. It is always less than or equal to 2-2, so it's negative for all x x .
  • Step 4: Establish the positive and negative domains
    Since the function is always negative, there are no positive domains. Therefore, the negative domain for the function is all real numbers x \>.

Therefore, the positive and negative domains are:

\( x > 0 : none

x<0: x < 0 : all x x

3

Final Answer

x>0: x > 0 : none

x<0: x < 0 : all x x

Key Points to Remember

Essential concepts to master this topic
  • Vertex Form: y=a(xh)2+k y = a(x - h)^2 + k reveals vertex at (h, k)
  • Sign Analysis: When a = -1, parabola opens downward with maximum at vertex
  • Domain Check: Compare function's maximum value -2 with zero to determine signs ✓

Common Mistakes

Avoid these frequent errors
  • Confusing positive/negative domains with function values
    Don't assume positive x means positive y values = wrong domain classification! The question asks where the function is positive or negative, not where x is positive or negative. Always analyze the function's output values, not input signs.

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 exactly are positive and negative domains?

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Positive domain: x-values where y > 0 (function output is positive)
Negative domain: x-values where y < 0 (function output is negative)

This is about the function's values, not whether x itself is positive or negative!

How do I know if a parabola opens up or down?

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Look at the coefficient a in y=a(xh)2+k y = a(x - h)^2 + k :

  • If a > 0: opens upward (smile shape)
  • If a < 0: opens downward (frown shape)

Here, a = -1, so it opens downward.

Why is the maximum value -2 and not something positive?

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The vertex is at (2619,2) (2\frac{6}{19}, -2) , which means the highest point the parabola reaches is y = -2. Since the parabola opens downward, all other points are even lower (more negative).

Could this function ever be positive anywhere?

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No! Since the maximum value is -2 and the parabola opens downward, every point on the graph has y ≤ -2. The function is always negative for all real numbers x.

What if the vertex was above the x-axis instead?

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If the vertex had a positive y-value (like y = 3), then the function would be positive near the vertex and negative farther away, giving us both positive and negative domains to identify.

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