June 2013 p12 q1
1246
A curve is such that \(\frac{dy}{dx} = \frac{6}{x^2}\) and \((2, 9)\) is a point on the curve. Find the equation of the curve.
Solution
Given \(\frac{dy}{dx} = \frac{6}{x^2}\), we need to find \(y\) by integrating \(\frac{dy}{dx}\).
Integrate \(\frac{dy}{dx} = \frac{6}{x^2}\):
\(y = \int \frac{6}{x^2} \, dx = \int 6x^{-2} \, dx\)
\(y = 6 \cdot \left( \frac{x^{-1}}{-1} \right) + c = -6x^{-1} + c\)
Using the point \((2, 9)\) to find \(c\):
\(9 = -6(2)^{-1} + c\)
\(9 = -3 + c\)
\(c = 12\)
Thus, the equation of the curve is \(y = -6x^{-1} + 12\).
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