## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$x^2-y^2$
We distribute to obtain: $$(x + y)(x - y)= x^2-xy+xy-y^2=x^2-y^2 .$$ Note that this is the difference of two squares formula: $(a-b)(a+b)=a^2-b^2$