Use Exercise 69 together with the identities sin (h + c) = sin h cos c + cos h sin c,...

Use Exercise 69 together with the identities

sin (h + c) = sin h cos c + cos h sin c,

cos (h + c) = cos h cos c - sin h sin c

to prove that both ƒ(x) = sin x and g(x) = cos x are continuous at every point x = c.

69. Prove that ƒ is continuous at c if and only if