已知sin(2x+y)=5siny,(y不等于K派),求证;3tan=2tan(x+y)
问题描述：
已知sin(2x+y)=5siny,(y不等于K派),求证;3tan=2tan(x+y)
答
sin[(x+y)+x]=5sin[(x+y)-x]
sin(x+y)·cosx＋cos(x+y)·sinx＝5·sin(x+y)·cosx－5·cos(x+y)·sinx
4·sin(x+y)·cosx＝6·cos(x+y)·sinx
两边同除以cos(x+y)·cosx得:2·tan(x+y)=3·tanx