prove that cos^3x+3cos2x= 4 (cos^6 x-sin^6x)

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17-05-2017
Mathematics

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prove that cos^3x+3cos2x= 4 (cos^6 x-sin^6x)

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LammettHash LammettHash · Answer 1 · 2017-05-17T16:56:33+02:00

LammettHash LammettHash

17-05-2017

But this isn't true... If [tex]x=\dfrac\pi2[/tex], you have

[tex]\cos^3\dfrac\pi2+3\cos\pi=0-3=-3[/tex]
[tex]4\left(\cos^6\dfrac\pi2-\sin^6\dfrac\pi2\right)=4(0-1)=-4[/tex]

but of course [tex]-3\neq-4[/tex].

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