Example 42

$\;\;\;(10x - y)^2$

$=(A - B)^2 \text{, where } A = 10x \text{ and } B = y $

$=A^2 - 2AB + B^2$

$=(10x)^2 - 2(10x)(y) + (y)^2 $

$=100x^{2} - 20xy +y^{2} $