how to prove? (cot^2 x-1) / (2 cot x) = cot 2x cos 3x = 4 cos^3 x-3 cosx sin 2t+sin t/1+cos t+sos 2t = tan t THANK YOU!
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Knowing the fact that cot x = 1/tan x .
Originally Posted by pongpong how to prove? cos 3x = 4 cos^3 x-3 cosx cos(2x+x)=cos2xcosx-sin2xsinx =(2cos^2x-1)cosx-2sinxcosx(sinx) =2cos^3x-cosx-2sin^2xcosx =2cos^3x-cosx-2(1-cos^2x)(cosx) =2cos^3x-cosx-2cosx+2cos^3x =4cos^3x-3cosx
Hello, pongpong! The third one is impossible to read. Well, not "impossible" . . . I figured it out! We're expected to know these Double-Angle Identities: . . . . . The left side becomes: . Factor and reduce: .
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