Finding Domains: y=-(x+7/8)² - 1⅕ Function Analysis

Quadratic Functions with Vertex Form Analysis

Find the positive and negative domains of the function below:

y=(x+78)2115 y=-\left(x+\frac{7}{8}\right)^2-1\frac{1}{5}

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Step-by-step written solution

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1

Understand the problem

Find the positive and negative domains of the function below:

y=(x+78)2115 y=-\left(x+\frac{7}{8}\right)^2-1\frac{1}{5}

2

Step-by-step solution

The function y=(x+78)2115 y = -\left(x + \frac{7}{8}\right)^2 - 1\frac{1}{5} is represented in vertex form, where the vertex of the parabola is at x=78 x = -\frac{7}{8} , and the maximum value at the vertex is y=115 y = -1\frac{1}{5} . Since the parabola opens downwards due to the negative coefficient of the squared term, its maximum value is also its highest possible value, which is a negative number 115 -1\frac{1}{5} .

Therefore, the function does not reach any positive value for any real number x x ; it only takes on non-positive values. Consequently, the determination of positive and negative domains is as follows:

  • x<0 x < 0 : The function can assume all values since the entire parabola lies below the x-axis, producing a negative range for all x x .
  • x>0 x > 0 : No x x can produce a positive y y value, resulting in no positive values.

Therefore, the positive and negative domains as concluded from this analysis are:

x<0: x < 0 : all x x

x>0: x > 0 : none

3

Final Answer

x<0: x < 0 : all x x

x>0: x > 0 : none

Key Points to Remember

Essential concepts to master this topic
  • Vertex Form: Identifies vertex and opening direction from coefficients
  • Maximum Value: y=115 y = -1\frac{1}{5} at vertex x=78 x = -\frac{7}{8}
  • Check Domain: All real x values give negative y outputs ✓

Common Mistakes

Avoid these frequent errors
  • Confusing domain with range when analyzing positive/negative regions
    Don't mix up domain (x-values) with range (y-values) = wrong positive/negative analysis! Students often think domain restrictions exist when the function is defined for all real numbers. Always remember domain asks 'what x-values work' while positive/negative regions ask 'where does y stay negative'.

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 does the parabola only have negative y-values?

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The vertex form shows the parabola opens downward (negative coefficient) with maximum value 115 -1\frac{1}{5} . Since even the highest point is negative, all y-values are negative!

What does 'positive and negative domains' actually mean?

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This asks: For which x-values does the function give positive y-values? and For which x-values does it give negative y-values? It's about the sign of the output, not input.

How do I find the vertex from this form?

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In y=(x+78)2115 y = -(x + \frac{7}{8})^2 - 1\frac{1}{5} , the vertex is at x=78 x = -\frac{7}{8} (opposite sign of what's inside) and y=115 y = -1\frac{1}{5} (the constant term).

Why is the answer 'all x' for negative domain but 'none' for positive?

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Since the maximum y-value is 115 -1\frac{1}{5} (negative), the function never reaches positive values. For any x-value you pick, y will always be negative!

Does the domain have any restrictions?

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No restrictions! This is a quadratic function defined for all real numbers. The domain is all real x-values, but the range (y-values) is limited to y115 y \leq -1\frac{1}{5} .

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