Formula

Extended Addition Property of Equality

The Extended Addition Property of Equality generalizes the basic Addition Property by allowing different but equal quantities to be added to the two sides of an equation, rather than requiring the same quantity on both sides. Formally, for any expressions aa, bb, cc, and dd:

If a=ba = b and c=dc = d, then a+c=b+da + c = b + d.

The basic Addition Property is a special case of this rule (where cc and dd happen to be the same value). The extended version is essential to the elimination method for solving systems of equations because it justifies adding the left-hand sides and right-hand sides of two different equations together: each equation is a statement of equality, so the two left sides and two right sides are equal quantities, and combining them preserves equality. For example, given 3x+y=53x + y = 5 and 2xy=02x - y = 0, the property guarantees that (3x+y)+(2xy)=5+0(3x + y) + (2x - y) = 5 + 0, which simplifies to 5x=55x = 5.

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Updated 2026-04-21

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