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 October 24th, 2017, 02:33 PM #1 Newbie   Joined: Oct 2017 From: Brazil Posts: 3 Thanks: 0 Solve the inequation Solve the inequation: cos*3 (a) . cos (3a) - sen*3 (a) . sen (3a) > 5/8
 November 9th, 2017, 04:11 AM #2 Math Team   Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 894 I recognize that "sen" would be, in English, "sin" but is "cos*3(a)" intended to be cos(a) to the third power (that I would write "cos^3(a)")? That is, is the inequality cos^3(a) cos(3a)- sin^3(a)sin(3a)> 5/8? If so, I would be inclined to use the facts that cos(3a)= cos^3(a)- 3cos(a)sin(a) and sin(3a)= sin(a)cos^2(a)- sin^3(a). Thanks from topsquark

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