(-3a^3)乘(-2a^2)=6a^51/（a+b）+2b/(a^2-b^2)=1/（a+b）+2b/[(a+b)(a-b)]
=(a-b+2b)/[(a+b)(a-b)]
=(a+b)/[(a+b)(a-b)]
=1/(a-b)