23. (a) Use the Constant Difference Theorem (3.8.3) to show that if f'(x) = g'(x) for all x in the interval (-co, too), and if f and g have the same value at some point to, then f(x) = g(x) for all x in (-0o, too). (b) Use the result in part (a) to confirm the trigonometric identity sin x + cos' x = 1