Question

[(x+y)(x-y)-(x-y)的平方+2y(x-y)]除4y

Answer 1

第一题=(x2-y2-x2+2xy-y2+2xy-2y2)／4y=(-4y2+4xy)／4y=x-y

第二题：（a+b)2=a2+2ab+b2=11 (a-b)2=a2-2ab+b2=7

两式向加得2（a2+b2)=18,即a2＋b2=9