Find f(α) and f(β)

Find \( f(\alpha) \) and \( f(\beta) \)

Question:

If

\[ f(x)=27x^3+\frac1{x^3} \]

and \(\alpha,\beta\) are roots of

\[ 3x+\frac1x=12, \]

then

(a) \(f(\alpha)\ne f(\beta)\)
(b) \(f(\alpha)=10\)
(c) \(f(\beta)=-10\)
(d) none of these

Solution:

Given,

\[ 3x+\frac1x=12 \]

Cubing both sides,

\[ \left(3x+\frac1x\right)^3=12^3 \]

\[ 27x^3+\frac1{x^3} +9\left(3x+\frac1x\right) =1728 \]

\[ f(x)+9(12)=1728 \]

\[ f(x)=1728-108 \]

\[ f(x)=1620 \]

Therefore,

\[ f(\alpha)=f(\beta)=1620 \]

None of the given options is correct.

\[ \boxed{\text{Correct Answer: (d)}} \]

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