sin^2A+sin^2C-sinAsinC=sin^2B由正玄定理原式转换为a^2+c^2-ac=b^2由余弦定理得cosB=(a^2+c^2-b^2)/(2ac)=[a^2+c^2-(a^2+c^2-ac)]/(2ac)=ac/(2ac)=1/2B=60°
