Definition of equivalent substitution: replacing another quantity (or part of another quantity) with one quantity (or part of another quantity) is a basic thinking method in mathematics and the basis of algebraic thinking method. The idea of equivalent substitution in a narrow sense embodies the essence of equality, that is, the transmission of equality.

Properties of equality

Property 1: add equal numbers or formulas on both sides of the equation. The two sides are still equal, just as the two ends of the balance are balanced, adding or subtracting the same weight items at both ends of the balance. The two ends of the balance are balanced.

Property 2: both sides of the equation multiply or divide equal and non-zero numbers or formulas at the same time. The two sides are still equal, just like an object whose two ends shrink or enlarge at the same time, the two ends of the balance keep balance.

Property 3: Both sides of the equation are multiplied or squared at the same time, and both sides are still equal. The items at both ends of the balance are multiplied or subtracted by half at the same time, and the two ends of the balance are balanced. This is the property of the equation.