8th Grade · Expressions & Equations
Substitution
8.EE.C.8b
Definition
A method for solving a system by replacing one variable with an equivalent expression from another equation, reducing it to a single-variable equation.
Examples
- y = 2x and x+y = 9: substitute to get x+2x = 9, so x = 3, y = 6.
- Best when a variable is already alone in one equation.
Key rule
Solve one equation for a variable, then substitute it into the other equation.
Memory hook
Swap one variable for its equivalent expression.