Example 25

$\;\;\;(x^{10} + 3y^{9})^2$

$=(A + B)^2 \text{, where } A = x^{10} \text{ and } B = 3y^{9} $

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

$=(x^{10})^2 + 2(x^{10})(3y^{9}) + (3y^{9})^2 $

$=x^{20} + 6x^{10}y^{9} +9y^{18} $