Use the result of Exercise 81 to show that (x) = 1/2 cos x has at most

Use the result of Exercise 81 to show that ƒ(x) = 1/2 cos x has at most one fixed point.

Data from in Exercise 81

Prove that if ƒ is differentiable on (-∞, ∞) and ƒ'(x) < 1 for all real numbers, then ƒ has at most one fixed point. A fixed point of a function ƒ is a real number c such that ƒ(c) = c.