1sin^2(A+B)=sinA^2+sinB^2,sin^2C=sin^2A+sin^2Ba/sinA=b/sinB=c/sinC=ksinA=a/k sinB=b/k sinC=c/kc^2/k^2=a^2/k^2+b^2/k^2c^2=a^2+b^2三角形ABC为直角三角形sinA^2+sinB^2=1 sin(A+B)=sinA^2+sinB^2=1 因...感谢第一题，好巧妙啊第二题x1+x2=（x1x2），不是x1+x2=（x1x2）/2