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Nov 2017 p12 q10
1280
The diagram shows part of the curve \(y = \sqrt{5x - 1}\) and the normal to the curve at the point \(P(2, 3)\). This normal meets the x-axis at \(Q\).
(i) Find the equation of the normal at \(P\).
(ii) Find, showing all necessary working, the area of the shaded region.
Solution
(i) To find the equation of the normal, first find the derivative of \(y = \sqrt{5x - 1}\). The derivative is \(\frac{dy}{dx} = \frac{1}{2}(5x - 1)^{-\frac{1}{2}} \times 5 = \frac{5}{2\sqrt{5x - 1}}\).
At \(P(2, 3)\), \(\frac{dy}{dx} = \frac{5}{2 \times 3} = \frac{5}{6}\).
The slope of the normal is the negative reciprocal: \(m = -\frac{6}{5}\).
The equation of the normal is \(y - 3 = -\frac{6}{5}(x - 2)\).
(ii) To find the area of the shaded region, integrate the curve from \(x = \frac{1}{5}\) to \(x = 2\):