Question
If x + 1x = 2cosθ, then x3 + 1x3 =
Answer: Option B
:
B
We have x + 1x = 2cosθ,
Now x3 + 1x3 = (x+1x)3 - 3x1x(x+1x)
= (2cosθ)3−3(2cosθ)=8cos3θ−6cosθ
=2(4cos3θ−3cosθ)=2cos3θ.
Trick:Put x = 1 ⇒ θ=0∘.
Then x3 + 1x3 = 2 = 2cos3θ.
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:
B
We have x + 1x = 2cosθ,
Now x3 + 1x3 = (x+1x)3 - 3x1x(x+1x)
= (2cosθ)3−3(2cosθ)=8cos3θ−6cosθ
=2(4cos3θ−3cosθ)=2cos3θ.
Trick:Put x = 1 ⇒ θ=0∘.
Then x3 + 1x3 = 2 = 2cos3θ.
Was this answer helpful ?
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