Put x = cos Ɵ, we get L.H.S = 3cos-1(cosθ) = 3θ As, &there4; R.H.S = cos-1(4(cos θ)3 - 3 cos θ) we know that by formula: cos3θ = 4cos3θ - 3cosθ L.H.S = R.H.S Hence, proved

Q. 54.3( 6 Votes )

Prove that

Class 12th Mathematics - Board Papers All India - 2018 (Compartment)

Answer :

Put x = cos Ɵ, we get

L.H.S = 3cos^-1(cosθ) = 3θ

As,

∴ R.H.S = cos^-1(4(cos θ)³ - 3 cos θ)

we know that by formula:

cos3θ = 4cos³θ - 3cosθ

L.H.S = R.H.S

Hence, proved

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