20. Euler’s formula gives a practical and easy way to derive many of the trigonometric identities involving the sine and cosine trigonometric functions. Verify the following

(Hint: eðaþbÞi ¼ eaiebi):

(a) cosða þ bÞ ¼ cos a cos b  sin a sin b

(b) sinða þ bÞ ¼ cos a sin b þ cos b sin a