3 (sin 10 + sin 80)(cos 80-cos 10)/2 sin 70

[latex] frac{3(sin10+sin80)(cos80-cos10)}{2sin70}=frac{3(sin10+sin(90-10))(cos(90-10)-cos10)}{2sin(90-20)}=\ frac{3(sin10+cos10)(sin10-cos10)}{2cos20}=frac{3(sin^210-cos^210)}{2cos20}=frac{-3cos(2*10)}{2cos20}=\-frac{3cos20}{2cos20}=- frac{3}{2}[/latex]