Finding X: Positive Values of a Parabola Intersection Problem

Quadratic Functions with Sign Analysis

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 x where f(x)>0 f\left(x\right) > 0 .

AAABBBCCCX

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Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

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 x where f(x)>0 f\left(x\right) > 0 .

AAABBBCCCX

2

Step-by-step solution

The graph of the parabola intersects the X-axis at points A and B. This tells us these are the roots of the quadratic equation, and that f(x)=0 f(x) = 0 at these points. Given that the shape of the parabola (concave up or down) affects where it is positive or negative:

From the graph:

  • If the parabola opens upwards (which it must, if we are finding f(x)>0 f(x) > 0 outside A and B), it is positive when x<A x \lt A or x>B x \gt B , as the parabola dips below the X-axis between A and B.
  • If the parabola opens downwards, it would be positive between A and B, however, our task is to identify the actual nature based on a graphical interpretation.

The graph signifies the function is positive outside the interval A<x<B A \lt x \lt B .

Therefore, the intervals where f(x)>0 f(x) > 0 are:

x>B x > B or x<A x < A

The answer choice that corresponds to this interpretation is:

x>B x > B or x<A x < A

3

Final Answer

x>B x > B or x<A x < A

Key Points to Remember

Essential concepts to master this topic
  • Zeros: Parabola crosses x-axis at points A and B
  • Sign Pattern: Upward parabola: negative between zeros, positive outside
  • Check: Pick test points: left of A, between A-B, right of B ✓

Common Mistakes

Avoid these frequent errors
  • Confusing where parabola is positive vs negative
    Don't assume f(x) > 0 between the zeros = wrong intervals! For upward-opening parabolas, the function dips below the x-axis between roots. Always remember: upward parabolas are positive outside the zeros, negative between them.

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

How do I know if the parabola opens upward or downward?

+

Look at the vertex position! If point C (vertex) is below the x-axis and A, B are on the x-axis, the parabola opens upward. The graph shows this clearly.

Why is f(x) > 0 outside the interval A to B?

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Since the parabola opens upward, it's like a U-shape. It starts above the x-axis (positive), dips down to cross at A, stays below the x-axis (negative) between A and B, then rises back up after B.

What if the parabola opened downward instead?

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If it opened downward (∩-shape), then f(x)>0 f(x) > 0 would be between A and B, and f(x)<0 f(x) < 0 would be outside that interval.

How can I verify my answer is correct?

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Pick test points! Choose any x-value less than A, between A and B, and greater than B. Check if the function is positive or negative at those points to confirm your intervals.

Do the exact values of A and B matter?

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No! What matters is the relationship between x and these points. Whether A = 2 and B = 5, or A = -1 and B = 3, the pattern stays the same for upward parabolas.

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