sin70(1-√3*tan50)
=sin70*[1+sin120*sin50/(cos120*cos50)]
=sin70*[(cos120*cos50+sin120sin50)/(cos120cos50)]
=sin70*cos(120-50)/(cos120cos50)
=sin70cos70/(cos120cos50)
=sin140/(2cos120sin40)
=1/(2cos120)
=-1