Activity
Access the Desmos guide PDF for tips on solving problems with the Desmos graphing calculator.
Julio is solving three equations by graphing.
To solve the first equation, , he graphed and then looked for the -intercepts of the graph.
1. Explain why the -intercepts can be used to solve .
Compare your answer:
The solutions to are -values that make the expression have a value of 0. At the -intercepts, the -value of the expression is 0, so the -intercepts give the solutions.
2. Graph . What are the solutions?
Compare your answer:
The solutions are and .
To solve the second equation, Julio rewrote it as . He then graphed .
3. Graph . Then, use the graph to solve the equation. Be prepared to explain how you used the graph for solving.
Compare your answer:
The solution is 4 for . The solution is the -coordinate of the -intercept.
Solve the third equation using Julio's strategy of graphing the equations.
4. What are the solutions to ?
Compare your answer:
There are no solutions for . The graph has no -intercept.
Think about the graphing strategy used to solve the equations and the solutions you found. View the Desmos graphs of all three equations.
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?
Compare your answer:
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 -intercepts of the graph of that function.
6. How many solutions does each of the three equations have?
Enter the number of solutions for each equation.
Compare your answer:
There are two solutions for the first equation, one solution for the second equation, and no solution for the last equation.
Are you ready for more?
Extending Your Thinking
- Graph and on the same coordinate plane.
How do the equations and help solve the equation ?
Compare your answer:
The solution to is the -value of the intersection point of the two graphs of and . This solution is 4.
- Graph and on the same coordinate plane.
How do the equations and help solve the equation ?
Compare your answer:
The solutions to are the -values of the intersection points of the two graphs of and . These solutions are 3 and 5.
- Graph and on the same coordinate plane.
How do the equations and help solve the equation ?
Compare your answer:
The solutions to are the -values of the intersection points of the two graphs of and . These solutions are 2 and 6.
4. Use the graphs you created in questions 1 – 3 to help you find a few other equations of the form that have whole-number solutions.
Compare your answer:
There are many different answers. For example:
,
5. Some of the values of that you might have found are –1, 0, 3, 8, 15, 24, etc. Find a pattern in the values of that give whole-number solutions.
Compare your answer:
Starting at , add 1, then 3, then 5, and so on, to find the next -value that yields whole-number solutions.
Because the expressions and represent numbers whose difference is 2, the values of have the form , , , , and so on.
6. Without solving, determine if and have whole-number solutions. Be prepared to show your reasoning.
Compare your answer:
Both have whole-number solutions. The values of that produce whole-number solutions are all 1 less than a square number.
Self Check
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 .
We will graph the equation .
This graph has two -intercepts.
The solutions to are -values that make the expression have a value of 0.
At the -intercepts, the -value of the expression is 0, so the -intercepts give the solutions.
The -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, -intercepts of the graph, roots of the equation, or zeros of the equation.
Example 2
Let's look at a different example.
First, we rewrite the equation to set it equal to 0.
We will graph the equation .
This graph has one -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 -coordinate of the -intercept.
So, the solution to is 5.5.
Example 3
Let's look at one last example.
First, we rewrite the equation to set it equal to 0.
We will graph the equation .
The graph has no -intercepts.
So, there are no real solutions for .
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 -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 . Then find any solutions.
Use the Desmos graphing tool or technology outside the course.
Compare your answer:
Here is how to analyze the graph of the equation to find any solutions:
Step 1 - Rewrite the equation to set it equal to 0.
Step 2 - Graph the equation .
Step 3 - Identify the -intercepts from the graph.
There is one -intercept at
This means there is one solution.
The solution is 6.5.