Lesson objective: Understand that properties of equality can be applied to write one variable equations that will solve for the values of variables in a system of equations.

Students bring prior knowledge of equivalent equations from 7.EE.A.2 and 7.EE.B.4. This prior knowledge is extended to systems of equations as students substitute equivalent terms and expressions to transform two variable equations into one variable equations and then, solve. A conceptual challenge students may encounter is replacing a variable with an expression rather than just a value.

The concept is developed through work with symbolic equations, which represent situations with equivalent quantities.

This work helps students deepen their understanding of operations and equivalence because properties of equality are used to write and solve equations by combining relationships in systems of equations.

Students engage in Mathematical Practice 2 (reason abstractly and quantitatively) as they reason about equivalent expressions and values to write and solve equations for values that will make both equations of a system true.

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