sinx=2sinx/2cosx/2=2[cos(x/2)]^2(tgx/2)=2tg(x/2)/[1+[tg(x/2)]^2]cosx=2(cosx/2)^2-1=2/[1+(tg(x/2)^2]-1=[1-[tg(x/2)]^2]/[1+[tg(x/2)]^2]tgx=2tg(x/2)/[1-[tg(x/2)]^2]