由正弦定理(sinA)^2=(sinB)^2+(sinC)^2等价于a^2=b^2+c^2可知△ABC直角三角形A=π/2sinA=2sinBcosC1=2sinBcos(π/2-B)1=2sinBsinBsinB=1/√2可知B=π/4△ABC等腰直角三角形
