Cos^ a - (1 - 2sin^2a)
= cos^ a - 1 + 2sin^2a
= cos^ a - 1 + 2(1 - cos^2a)
= cos^ a - 1 + 2 - 2cos^2a
= -1 - cos^2a

Therefore, Cos^ a - (1-2sin^2a) simplifies to -1 - cos^2a.