Thursday, 10 July 2014

trigonometry - Prove that cos6betasin6beta=frac116(15cos2beta+cos6beta).




Prove the given trigonometric identity:
cos6βsin6β=116(15cos2β+cos6β)





My Approach:
L.H.S.=cos6βsin6β=(cos2β)3(sin2β)3=cos32β+3cos2βsin2βcos2β



Please help me to continue further.


Answer




(cos2β)3(sin2β)3=(cos2βsin2β)(cos4β+sin4β+cos2βsin2β)
=cos2β(1sin2βcos2β)
=cos2β(1sin22β4)
=cos2β(1+cos4β18)
Now use 2cosA.cosB=cos(A+B)+cos(AB)
=18(7cos2β+cos2βcos4β)
=18(7cos2β+cos6β+cos2β2)
=116(15cos2β+cos6β)


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