# 7.4: Solving Systems of Equations by Multiplication

**At Grade**Created by: CK-12

## Learning Objectives

At the end of this lesson, students will be able to:

- Solve a linear system by multiplying one equation.
- Solve a linear system of equations by multiplying both equations.
- Compare methods for solving linear systems.
- Solve real-world problems using linear systems by any method.

## Vocabulary

Terms introduced in this lesson:

- scalar
- lowest common multiple

## Teaching Strategies and Tips

Use the introduction to convince students that the elimination method applies to *any* linear system because one or both equations can be multiplied by a constant resulting in a “new” pair of equations with matching coefficients.

In Example 1, some students have trouble keeping track of the multipliers for each equation. Try using a visual:

In Example 2,

- Point out that the distance covered is the same in both directions, so the
400yards is unnecessary information. - Remind students to back-substitute to complete the problem.

Use Example 3 to demonstrate a system with no matching coefficients and no coefficients that are multiples of others.

- Remind students how to find the LCM of two numbers.
- Suggest that students find the LCM of the “smaller pair” of coefficients,
3 and5 , instead of880 and 1845.

Teachers may decide to forgo back-substitution and instead teach elimination of the second variable (“double elimination”).

Additional Example:

*Solve the system using multiplication.*

Solution by “double elimination”. Eliminate

Add.

Divide by

To find

Add.

Divide by

Answer: The solution to the system is

The advantage of “double elimination” is that the fraction does not need to be back-substituted.

## Error Troubleshooting

General Tip: Students forget to multiply *every* term in an equation by the scalar.

General Tip: Encourage students to eliminate the variable whose coefficients in both equations have the smallest LCM.

Remind students in *Review Problem* 2e to align the variables column-wise.

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