利用和差化积得来的sinB+sin(2π/3-B)=3/22sin[B+2π/3-B]/2*cos[B-(2π/3-B)]/2=3/22sinπ/3*cos(B-π/3)=3/2√3cos(B-π/3)=3/2√3cos(π/3-B)=3/2√3cos[π/2-(π/3-B)]=3/2√3sin(π/6+B)=3/2
