(a) Use your calculator to evaluate the expression cos 72
Chapter 8, Problem 59(choose chapter or problem)
(a) Use your calculator to evaluate the expression cos 72 cos 144. Then follow steps (b) through (d) to prove that cos 72 cos 1441/2. (b) Use the observation cos 72 cos 144 cos(108 36) cos(108 36) and the addition formulas for cosine to show that cos 72 cos 144 2 cos 108 cos 36 1 4 s(c) Use the reference-angle concept to explain whycos 108 cos 36 cos 72 cos 144.(d) Use parts (b) and (c) and the identity in Exercise 58(a)to conclude that cos 72 cos 1441/2, as required.
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