Nov 2020 p13 q10
1097
A curve has equation \(y = \frac{1}{k}x^{\frac{1}{2}} + x^{-\frac{1}{2}} + \frac{1}{k^2}\) where \(x > 0\) and \(k\) is a positive constant.
It is given that when \(x = \frac{1}{4}\), the gradient of the curve is 3.
Find the value of \(k\).
Solution
To find the gradient of the curve, we need to differentiate the given equation:
\(y = \frac{1}{k}x^{\frac{1}{2}} + x^{-\frac{1}{2}} + \frac{1}{k^2}\).
The derivative \(\frac{dy}{dx}\) is:
\(\frac{dy}{dx} = \frac{1}{2k}x^{-\frac{1}{2}} - \frac{1}{2}x^{-\frac{3}{2}}\).
Substitute \(x = \frac{1}{4}\) into the derivative:
\(\frac{dy}{dx} = \frac{1}{2k}(\frac{1}{4})^{-\frac{1}{2}} - \frac{1}{2}(\frac{1}{4})^{-\frac{3}{2}}\).
Simplify \((\frac{1}{4})^{-\frac{1}{2}} = 2\) and \((\frac{1}{4})^{-\frac{3}{2}} = 8\):
\(\frac{dy}{dx} = \frac{1}{2k} \cdot 2 - \frac{1}{2} \cdot 8\).
\(\frac{dy}{dx} = \frac{1}{k} - 4\).
It is given that \(\frac{dy}{dx} = 3\) when \(x = \frac{1}{4}\):
\(3 = \frac{1}{k} - 4\).
Solving for \(k\):
\(3 + 4 = \frac{1}{k}\).
\(7 = \frac{1}{k}\).
\(k = \frac{1}{7}\).
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