The function \(f\) is defined by \(f(x) = x^2 + \frac{k}{x} + 2\) for \(x > 0\).
(a) Given that the curve with equation \(y = f(x)\) has a stationary point when \(x = 2\), find \(k\).
(b) Determine the nature of the stationary point.
(c) Given that this is the only stationary point of the curve, find the range of \(f\).
Solution
(a) To find \(k\), first find the derivative \(f'(x) = 2x - \frac{k}{x^2}\).
Set \(f'(2) = 0\):
\(2 \times 2 - \frac{k}{2^2} = 0\)
\(4 - \frac{k}{4} = 0\)
\(k = 16\)
(b) To determine the nature of the stationary point, find the second derivative \(f''(x) = 2 + \frac{2k}{x^3}\).
Evaluate \(f''(2)\):
\(f''(2) = 2 + \frac{2 \times 16}{2^3} = 2 + \frac{32}{8} = 6\)
Since \(f''(2) > 0\), the stationary point is a minimum.
(c) When \(x = 2\), \(f(x) = 2^2 + \frac{16}{2} + 2 = 4 + 8 + 2 = 14\).
Since this is the only stationary point and it is a minimum, the range of \(f\) is \(y \geq 14\).
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