(a) Use two of the addition formulas from the previous
Chapter 8, Problem 65(choose chapter or problem)
(a) Use two of the addition formulas from the previous section to show that cos(60 u) cos(60 u) (4 cos2 u 3)/4 (b) Show that cos u cos(60 u) cos(60 u) (cos 3u)/4 Hint: Use the result in part (a) and the identity for cos 3u that was derived in Example 3 on page 577. (c) Use the result in part (b) to show that cos 20 cos 40 cos 80 1/8 (d) Use a calculator to check that cos 20 cos 40 cos 80 1/8 6
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