cos²(A/2)=(1/2)[cosA＋1]=(sinB＋sinC)/2sinC,即：
sinC(cosA＋1)=sinB＋sinC=sin(A＋C)＋sinC
sinCcosA＋sinC=sinAcosC＋cosAsinC＋sinC
sinAcosC=0
从而C=90°.