x-y/x+3y-x的平方-9y的平方/x的平方+6xy+9y的平方+2y/x+3y,其中x=2,y=-3 ……
问题描述：
x-y/x+3y-x的平方-9y的平方/x的平方+6xy+9y的平方+2y/x+3y,其中x=2,y=-3 ……
答
原式=(x-y)/(x+3y)-(x+3y)(x-3y)/(x+3y)²+2y/(x+3y)
=(x-y)/(x+3y)-(x-3y)/(x+3y)+2y/(x+3y)
=(x-y-x+3y+2y)/(x+3y)
=4y/(x+3y)
=-12/(2-9)
=12/7