cosB=1/2 sinB=√3/2
sinC=3/5cosC=4/5
sinA=sin(B+C)=sinBcosC+cosBsinC=3√3/10+4/10=(4+3√3)/10要的是余弦cosA=-cos(B+C)=-[cosAcosB-sinAsinB]=-[4/10-3√3/10]=(3√3-4)/10