June 2010 p13 q5
1397
The equation of a curve is such that \(\frac{dy}{dx} = \frac{6}{\sqrt{3x - 2}}\). Given that the curve passes through the point \(P(2, 11)\), find
(i) the equation of the normal to the curve at \(P\),
(ii) the equation of the curve.
Solution
(i) At \(x = 2\), the tangent has gradient \(3\). Therefore, the normal has gradient \(-\frac{1}{3}\) since \(m_1 m_2 = -1\).
The equation of the normal is \(y - 11 = -\frac{1}{3}(x - 2)\).
(ii) To find the equation of the curve, integrate \(\frac{dy}{dx} = \frac{6}{\sqrt{3x - 2}}\).
Integrate: \(y = 6 \int \frac{1}{\sqrt{3x - 2}} \, dx\).
This gives \(y = 4\sqrt{3x - 2} + c\).
Using the point \((2, 11)\) to find \(c\):
\(11 = 4\sqrt{3(2) - 2} + c\)
\(11 = 4\sqrt{4} + c\)
\(11 = 8 + c\)
\(c = 3\)
Thus, the equation of the curve is \(y = 4\sqrt{3x - 2} + 3\).
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