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Nov 2009 p11 q4
1305
The equation of a curve is \(y = x^4 + 4x + 9\).
(i) Find the coordinates of the stationary point on the curve and determine its nature.
(ii) Find the area of the region enclosed by the curve, the x-axis and the lines \(x = 0\) and \(x = 1\).
Solution
(i) To find the stationary point, we first find the derivative of the curve: \(\frac{dy}{dx} = 4x^3 + 4\). Setting this to zero gives \(4x^3 + 4 = 0\), which simplifies to \(x^3 = -1\), so \(x = -1\). Substituting \(x = -1\) back into the original equation gives \(y = (-1)^4 + 4(-1) + 9 = 1 - 4 + 9 = 6\). Thus, the stationary point is \((-1, 6)\).
To determine the nature, we find the second derivative: \(\frac{d^2y}{dx^2} = 12x^2\). Substituting \(x = -1\) gives \(12(-1)^2 = 12\), which is positive, indicating a minimum point.
(ii) To find the area, we integrate the curve from \(x = 0\) to \(x = 1\). The integral of \(y = x^4 + 4x + 9\) is \(\int (x^4 + 4x + 9) \, dx = \frac{x^5}{5} + 2x^2 + 9x\). Evaluating from 0 to 1 gives: