Cos(π/6-a)-cos(π/6+a) =sina

Question

Cos(π/6-a)-cos(π/6+a) =sina

Answer 1

-2sin[(π/6-a-π/6-a)/2]*sin[(π/6-a+π/6+a)/2]=sina
-2sin(-a)*sinπ/6=sina
-2*(-sina)*1/2=sina
sina=sina