In the process of change, first consider the sine theorem, which is divided into two kinds of changes, one is the corner and the other is the keratinized edge; Whether the sine theorem is applicable or not depends on whether 2R can be eliminated after the change. The complementary angle of this problem can be eliminated by 2R, so it is converted into (Sina) 2+(sinb) 2 = sinc by sine theorem. If a+b > π/2, then sinA>cosB, sinB>cosA,

∴sin2a+sin2b>sinacosb+cosasinb=sin(a+b)=sinc，

This contradicts asinA+bsinB=c,

Similarly, a+b < π 2 is also impossible.

∴A+B=π2，

∴∠C=90。