Namely: cos2a-cos2b = 3 (1/2) (sin2a-sin2b)

3^( 1/2)sin2a-cos2a=3^( 1/2)sin2b-cos2b

2 symplectic (2A-π/6)= 2 symplectic (2B-π/6)

Another reason: a≠b

So: ∠ A ≦ B.

So: 2A-π/6=π-(2B-π/6)

So: A+B=2π/3

So: ∠C=π/3

I hope it helps you.