algebra 1 Lesson

Elimination: Making a Variable Disappear

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8th gradeBTC Style / Thinking ClassroomsCCSS.MATH.CONTENT.8.EE.C.8b, CCSS.MATH.CONTENT.HSA.REI.C.550 min

Learning Objective

Students will solve systems of two linear equations in two variables using the elimination method, including cases requiring multiplication of one or both equations by a constant.

Lesson Overview

Students discover that adding or subtracting two equations can eliminate a variable, then progressively tackle systems requiring multiplication before elimination, building to systems with fractional or decimal coefficients.

Materials

Vertical whiteboards or chart paper
Dry-erase markers
Markers in two colors (optional)
Random grouping tool
Four-quadrant note sheets (one per student)
Exit slip half-sheets

Scaffolded Task Progression

1Task 1

Two equations are written below. Add them together (combine like terms on each side). What do you notice?

x + y = 10

x - y = 4

What is the sum equation? Can you solve for

x

? Then find

y

2Task 2

Now try subtracting the second equation from the first. What variable disappears?

3 x + y = 11

x + y = 5

Solve the system.

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