sin（x+y）=sinxcosy+cosxsiny=1/3,sin（x-y）=sinxcosy-cosxsiny=1/5,两式作商得(sinxcosy+cosxsiny)/(sinxcosy-cosxsiny)=5/3,分子分母同时除以cosxcosy得(tanx+tany)/(tanx-tany)=5/3,所以3tanx+3tany=5tanx-5tan...