求证:sin(2x+y)/sinx-2cos(x+y)=siny/sinx(2)已知5siny=sin(2x+y),求证:tan(x+y)=3/2 tanx

问题描述:

求证:sin(2x+y)/sinx-2cos(x+y)=siny/sinx
(2)已知5siny=sin(2x+y),求证:tan(x+y)=3/2 tanx

把5siny=sin(2x+y)变为5sin[(x+y)-x]=sin[(x+y)+x],把其中的(x+y),看成一个整体,上式即变为4sin(x+y)cosx=6cos(x+y)sinx,再把式子的左右两边变换为sin(x+y)/cos(x+y)=3/2sinx/cosx,即tan(x+y)=3/2 tanx