x的-2次方-y的-2次方分之x的-1次方-y的-1次方
=[(x的-1次方)²-(y的-1次方)²]分之(x的-1次方-y的-1次方)
=(x的-1次方+y的-1次方)(x的-1次方-y的-1次方)分之(x的-1次方-y的-1次方)
=(x的-1次方+y的-1次方)分之1
=(x+y)分之xy