Question: Use trigonometric and hyperbolic identities to solve trigonometric equations and to simplify trigonometric expressions

a) \[\cos 7\omega t\,\cos 3\omega t+\sin 7\omega t\,\sin 3\omega t\]

b) \[\sin 2\omega t\,\,\cos (2\omega t-k)-\cos 2\omega t\,\,\sin (2\omega t-k)\]

c) \[{{\cosh }^{2}}A-1\] (use exponential representation)