How to prove : \cos^3 2\theta+3\cos 2\theta=4(\cos^6 \theta−\sin^6 \theta)

Jenny Branch

Jenny Branch

Answered question

2022-01-23

How to prove : cos32θ+3cos2θ=4(cos6θsin6θ)

Answer & Explanation

Eleanor Shaffer

Eleanor Shaffer

Beginner2022-01-24Added 16 answers

Write cos2θ=x. We have
x=2cos2θ1=12sin2θ
1+x2=cos2θ
1x2=sin2θ
Thus the RHS becomes
4((1+x)38(1x)38)=12(1+3x+3x2+x3(13x+3x2x3))=x3+3x=LHS
chaloideq1

chaloideq1

Beginner2022-01-25Added 11 answers

4(cos6θsin6θ)=4(cos2θsin2θ)(cos4θ+cos2θsin2θ+sin4θ)
=4(cos2θsin2θ)((cos2θ+sin2θ)22cos2θsin2θ+cos2θsin2θ)
=4(cos2θsin2θ)(1cos2θsin2θ)=4cos2θ(1sin22θ4)=4cos2θsin22θcos2θ
=4cos2θ(1cos22θ)cos2θ=3cos2θ+cos3θ

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