1.sinA=4/5,cosB=12/13
cosC=cos(pi-(A+B))
=-cos(A+B)
=-(cosAcosB-sinAsinB)
=3/5*12/13-4/5*5/13
=-16/65
2.S==(1/2)*absinC
=(1/2)*cos40°*cos80°*sin70°
=(1/2)*cos20°*cos40°*cos80°
=(1/2)*sin20°*cos20°*cos40°*cos80°/sin20°
=(1/2)*(1/8)*sin160°/sin20°
因为sin160°=sin20°
所以S=1/16