Question

(x+3y)^2-2(x^2-9y^2)+(x-3y)^2

Answer 1

(x+3y)^2-2(x^2-9y^2)+(x-3y)^2

=（x+3y）^2-2(x+3y)(x-3y)+(x-3y)^2

=(x+3y-x+3y)^2 (应用平方和公式)

=(6y)^2

=36y^2