tan(x+y)tan(x-y)=sin^2x-sin^2y/cos^2x-sin^2y 顺便问一下. tan,sin,cos是什么意思?

tan,正切；sin,正弦；cos,余弦tan(x+y)tan(x-y)=sin(x+y)/cos(x+y) * sin(x-y)/cos(x-y)=sin(x+y)sin(x-y) / [cos(x+y)cos(x-y)]=(sinxcosy+cosxsiny)(sinxcosy-cosxsiny) / [(cosxcosy-sinxsiny)(cosxcosy+sinxsiny...