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

$x^2-4xy+4y^2$
We square (expand) the binomial to obtain: $$(x - 2y)^2= (x-2y)(x-2y)=x^2+2\cdot x \cdot (-2y)+(-2y)^2=x^2-4xy+4y^2 .$$