18(x+y)^8(x-y)^7÷[3(x+y)^3(x-y)^3]^2
= 18(x+y)^8(x-y)^7÷[9(x+y)^6(x-y)^6]
= 2(x+y)^2(x-y)
用到公式：
(a^m)^n = a^(mn)
a^m/a^n = a^(m-n)