sin^2(A+B)=sinA^2+sinB^2,
sin^2C=sin^2A+sin^2B
a/sinA=b/sinB=c/sinC=k
sinA=a/k sinB=b/k sinC=c/k
c^2/k^2=a^2/k^2+b^2/k^2
c^2=a^2+b^2
三角形ABC为直角三角形