已知tan(x+y)=2/5,tan(x-y)=-1/4,则tan2x的值是
问题描述：
已知tan(x+y)=2/5,tan(x-y)=-1/4,则tan2x的值是
答
tan2x=tan [(x+y)+(x-y)]
=[tan(x+y)+tan(x-y)]/[1-tan(x+y)tan(x-y)]
=3/22