McGraw Hill Integrated II, 2012
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McGraw Hill Integrated II, 2012 View details
2. Solving Quadratic Equations by Graphing
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Exercise 2 Page 108

Make sure the equation is written in standard form. Identify the related function and graph it.

Graph:

Solutions: 0, 4

Practice makes perfect

We are asked to solve the given quadratic equation. We will solve it by graphing. To solve a quadratic equation by graphing, we have to follow three steps.

  1. Write the equation in standard form, ax^2+bx+c=0.
  2. Graph the related function f(x)=ax^2+bx+c.
  3. Find the x-intercepts, if any.
The solutions, or roots, of ax^2+bx+c=0 are the x-intercepts of the graph of f(x)=ax^2+bx+c. Now we can identify the function related to the equation. Equation:& 2x^2-8x=0 Related Function:& f(x)=2x^2-8x

Graphing the Related Function

To draw the graph of the related function written in standard form, we must start by identifying the values of a, b, and c. f(x)=2x^2-8x ⇕ f(x)= 2x^2+( - 8)x+ 0 We can see that a= 2, b= - 8, and c= 0. Now, we will follow three steps to graph the function.

  1. Find the axis of symmetry.
  2. Make a table of values using x-values around the axis of symmetry.
  3. Plot and connect the points with a parabola.

Finding the Axis of Symmetry

The axis of symmetry is a vertical line with equation x=- b2 a. Since we already know the values of a and b, we can substitute them into the formula.
x=- b/2a
x=- - 8/2( 2)
â–Ľ
Evaluate right-hand side
x=-- 8/4
x=8/4
x=2
The axis of symmetry of the parabola is the vertical line with equation x=2.

Making the Table of Values

Next, we will make a table of values using x-values around the axis of symmetry x=2.

x 2x^2-8x f(x)
- 2 2( - 2)^2-8( - 2) 24
0 2( 0)^2-8( 0) 0
2 2( 2)^2-8( 2) - 8
4 2( 4)^2-8( 4) 0
6 2( 6)^2-8( 6) 24

Plotting and Connecting the Points

We can finally draw the graph of the function. Since a= 2, which is positive, the parabola will open upwards. Let's connect the points with a smooth curve.

Finding the x-intercepts

Let's identify the x-intercepts of the graph of the related function.

We can see that the parabola intersects the x-axis twice. The points of intersection are ( 0,0) and ( 4,0). Therefore, the equation 2x^2-8x=0 has two solutions, x= 0 and x= 4.