sin20градусов = sin(90 - 70) = cos70
cos70*sin70 = 1/2*sin140
sin20*sin70=sin20*cos(90-70)=sin20*cos20=2sin20*cos20/2=sin40/2
sin20градусов = sin(90 - 70) = cos70
cos70*sin70 = 1/2*sin140
sin20*sin70=sin20*cos(90-70)=sin20*cos20=2sin20*cos20/2=sin40/2