Math Problem Statement

Find the equation of the quadratic curve given its graph with x-intercepts at x = -1 and x = 3, and a vertex at (1, -9).

Solution

The graph represents a quadratic curve, and you are asked to find the equation of the curve in the form:

y=ax2+bx+cy = ax^2 + bx + c

We are given the following key points from the graph:

  1. The x-intercepts are x=1x = -1 and x=3x = 3, which gives the roots of the quadratic equation.
  2. The vertex is at (1,9)(1, -9).

Step 1: Form the general equation

Since the quadratic equation has roots at x=1x = -1 and x=3x = 3, we can write it in its factored form as:

y=a(x+1)(x3)y = a(x + 1)(x - 3)

Here, aa is a constant that needs to be determined.

Step 2: Use the vertex to find aa

The vertex of the parabola is given as (1,9)(1, -9), meaning when x=1x = 1, y=9y = -9. We can substitute x=1x = 1 and y=9y = -9 into the equation to find the value of aa.

9=a(1+1)(13)-9 = a(1 + 1)(1 - 3) 9=a(2)(2)-9 = a(2)(-2) 9=4a-9 = -4a a=94=94a = \frac{-9}{-4} = \frac{9}{4}

Thus, the equation of the quadratic becomes:

y=94(x+1)(x3)y = \frac{9}{4}(x + 1)(x - 3)

Step 3: Expand the equation

Now, expand the equation to get it in the form y=ax2+bx+cy = ax^2 + bx + c.

y=94(x23x+x3)y = \frac{9}{4}(x^2 - 3x + x - 3) y=94(x22x3)y = \frac{9}{4}(x^2 - 2x - 3) y=94x2184x274y = \frac{9}{4}x^2 - \frac{18}{4}x - \frac{27}{4}

Finally, we multiply through by 4 to clear the fractions:

4y=9x218x274y = 9x^2 - 18x - 27

So, the equation of the curve is:

y=94x2184x274y = \frac{9}{4}x^2 - \frac{18}{4}x - \frac{27}{4}

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Math Problem Analysis

Mathematical Concepts

Quadratic Functions
Factored Form
Vertex

Formulas

Factored form of quadratic: y = a(x - p)(x - q)
Vertex form: y = ax^2 + bx + c

Theorems

Roots of a quadratic equation
Vertex formula and properties

Suitable Grade Level

Grades 9-12