sin3a-sin9a=2sin(-3a)*cos6a=-2sin3acos6a
cos13x+cos12x=2cos(12.5x)cos(x/2)
tg5П/8-ctgП/8=-ctgП/8-ctgп/8=-2ctgП/8=-2sinП/4/(1-cosП/4)=2sqrt(2)/(sqrt(2)-2)
tg5x+ctg8x=cos3x/(cos5xsin8x)
(cosa+sina)/(cosa-sina)=(1+tga)/(1-tga)
(sin40+sin20)/(sin40-sin20)=2sin30cos10/2sin10cos30=tg30/tg10=sqrt(3)ctg10/3