很简单啊,xy都扩大两倍,那么xy/(x+y)就变成了4xy/2(x+y),把扩大了的这个数去除以原来的xy/(x+y),就是:
[4xy/2(x+y)]/[xy/(x+y)]
=[4xy/2(x+y)]*[(x+y)/xy]
=4/2
=2