sinA*sin(B-C)=sinC*sin(A-B)，

SinA=sin(PI-B-C)=sin(B+C), sinC=sin(PI-A-B)=sin(A+B), and the above formula can be changed to: sin (b+c) * sin (b-c) = sin (a+b) * sin (a

(sinb) 2-(sinc) 2 = (sina) 2-(sinb) 2 By merging and transposition, we can get:

2*(sinB)^2=(sinA)^2+(sinC)^2

Available: 2b 2 = a 2+c 2

So: A 2, B 2 and C 2 are arithmetic progression.