(a) Use your calculator to evaluate the expression cos 72
Chapter 8, Problem 58(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 1441/4. (b) Multiply the expression cos 72 cos 144 by the quantity (sin 72)/(sin 72), which equals 1. Show that the result can be written (c) Explain why the expression obtained in part (b) is equal to (d) Use the reference-angle concept to explain why the expression in part (c) is equal to 1/4, as required.
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