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Question

If cos θ = 35 and cos ϕ = 45, where θ and ϕ are

positive acute angles, then cos θϕ2 =


A

72

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B

752

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C

75

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D

725

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Solution

The correct option is B

752


We have cos θ = 35 and cos ϕ = 45.

Therefore cos(θϕ)=cosθcosϕ+sinθsinϕ

= 35 . 45 + 45 . 35 = 2425

But 2cos2(θϕ2)=1+cos(θϕ) = 1 + 2425 = 4950

cos2(θϕ2) = 4950. Hence, cos(θϕ2) = 752.


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