a=2RsinA b=2RsinB c=2RsinCsin^2A=sin^2B+sin^2Ca^2=b^2+C^2A=90°（a+b+c)(a+b-c)=3aba^2+b^2-C^2-ab=0将C^2=a^2-b^2代入上式2b^2-ab=0b≠0a=2bA=90° B=30° C=60°
