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Algebra 1

8.5.2 Interpreting Graphs to Solve Quadratic Equations

Algebra 18.5.2 Interpreting Graphs to Solve Quadratic Equations

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Activity

Access the Desmos guide PDF for tips on solving problems with the Desmos graphing calculator.

Julio is solving three equations by graphing.

(x5)(x3)=0(x5)(x3)=0

(x5)(x3)=1(x5)(x3)=1

(x5)(x3)=4(x5)(x3)=4

To solve the first equation, (x5)(x3)=0(x5)(x3)=0, he graphed y=(x5)(x3)y=(x5)(x3) and then looked for the xx-intercepts of the graph.

1. Explain why the xx-intercepts can be used to solve (x5)(x3)=0(x5)(x3)=0.

2. Graph y=(x5)(x3)y=(x5)(x3). What are the solutions?

To solve the second equation, Julio rewrote it as (x5)(x3)+1=0(x5)(x3)+1=0. He then graphed y=(x5)(x3)+1y=(x5)(x3)+1.

3. Graph y=(x5)(x3)+1y=(x5)(x3)+1. Then, use the graph to solve the equation. Be prepared to explain how you used the graph for solving.

Solve the third equation using Julio's strategy of graphing the equations.

4. What are the solutions to y=(x5)(x3)+4y=(x5)(x3)+4?

Think about the graphing strategy used to solve the equations and the solutions you found. View the Desmos graphs of all three equations.

A graph with three upward-opening parabolas in red, blue, and green on a grid. All have similar shapes and are centered around x equals 4, but each reaches its minimum at different y-values.

5. Why might it be helpful to rearrange each equation to equal 0 on one side and then graph the expression on the non-zero side?

6. How many solutions does each of the three equations have?

Are you ready for more?

Extending Your Thinking

1.
  1. Graph y=(x3)(x5)y=(x3)(x5) and y=1y=1 on the same coordinate plane.

How do the equations y=(x3)(x5)y=(x3)(x5) and y=1y=1 help solve the equation (x3)(x5)=1(x3)(x5)=1?

2.
  1. Graph y=(x3)(x5)y=(x3)(x5) and y=0y=0 on the same coordinate plane.

How do the equations y=(x3)(x5)y=(x3)(x5) and y=0y=0 help solve the equation (x3)(x5)=0(x3)(x5)=0?

3.
  1. Graph y=(x3)(x5)y=(x3)(x5) and y=3y=3 on the same coordinate plane.

How do the equations y=(x3)(x5)y=(x3)(x5) and y=3y=3 help solve the equation (x3)(x5)=3(x3)(x5)=3?

4.

4. Use the graphs you created in questions 1 – 3 to help you find a few other equations of the form (x3)(x5)=z(x3)(x5)=z that have whole-number solutions.

5.

5. Some of the values of zz that you might have found are –1, 0, 3, 8, 15, 24, etc. Find a pattern in the values of zz that give whole-number solutions.

6.

6. Without solving, determine if (x5)(x3)=120(x5)(x3)=120 and (x5)(x3)=399(x5)(x3)=399 have whole-number solutions. Be prepared to show your reasoning.

Self Check

Use graphing technology to determine the number of solutions for the equation ( x + 3 ) ( x 6 ) = 22 . Then find any solutions.
  1. The equation has no solutions.
  2. There are two solutions. x = 3 or x = 6
  3. There are two solutions. x = 6 or x = 3
  4. There is one solution. x = 5.5

Additional Resources

Interpreting Graphs to Solve Quadratic Equations

Let's analyze and interpret the graphs of different quadratic equations.

Example 1

First, let's look at the equation (x4)(x7)=0(x4)(x7)=0.

We will graph the equation y=(x4)(x7)y=(x4)(x7).

Graph of a parabola that opens up and passes through the points (4, 0) and (7, 0).

This graph has two xx-intercepts.

The solutions to (x4)(x7)=0(x4)(x7)=0 are xx-values that make the expression have a value of 0.

At the xx-intercepts, the yy-value of the expression (x4)(x7)(x4)(x7) is 0, so the xx-intercepts give the solutions.

The xx-intercepts and solutions to the equation are 4 and 7.

Note that the values of 4 and 7 could be referred to as solutions of the equation, xx-intercepts of the graph, roots of the equation, or zeros of the equation.

Example 2

Let's look at a different example.

(x4)(x7)=2.25(x4)(x7)=2.25

First, we rewrite the equation to set it equal to 0.

(x4)(x7)+2.25=0(x4)(x7)+2.25=0

We will graph the equation y=(x4)(x7)+2.25y=(x4)(x7)+2.25.

Graph of a parabola that opens up with a vertex at (5.5, 0).

This graph has one xx-intercept. It is an example of a double root at 5.5. A double root is a root that appears twice in the solution of an algebraic equation.

The solution is the xx-coordinate of the xx-intercept.

So, the solution to (x4)(x7)=2.25(x4)(x7)=2.25 is 5.5.

Example 3

Let's look at one last example.

(x4)(x7)=5(x4)(x7)=5

First, we rewrite the equation to set it equal to 0.

(x4)(x7)+5=0(x4)(x7)+5=0

We will graph the equation y=(x4)(x7)+5y=(x4)(x7)+5.

Graph of a parabola that opens up the graph does not touch or cross the x-axis the graph passes through the points (4, 5) and (7, 5).

The graph has no xx-intercepts.

So, there are no real solutions for (x4)(x7)+5=0(x4)(x7)+5=0.

Remember, when a quadratic expression is equal to 0 and we want to find the unknown values, we can think of it as finding the zeros of a quadratic function, or finding the xx-intercepts of the graph of that function.

Try it

Try It: Interpreting Graphs to Solve Quadratic Equations

Determine the number of solutions for the equation (x4)(x9)=6.25(x4)(x9)=6.25. Then find any solutions.

Use the Desmos graphing tool or technology outside the course.

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