Verified answer

  cos(a+b) • cos(a+b) = cos²(a) – sin²(b) ... Is this true ???

       2  cos(a+b) = cos²(a) – sin²(b)

... when a = b = x

         2  cos(2a) = cos²(a) – sin²(a)

... based on the identity:  cos(2x) = cos²(x) − sin²(x)

         2  cos(2a) ≠ cos²(a) – sin²(a) ... NOT PROVED