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June 2018 p11 q10
1062
The curve with equation \(y = x^3 - 2x^2 + 5x\) passes through the origin.
(i) Show that the curve has no stationary points.
(ii) Denoting the gradient of the curve by \(m\), find the stationary value of \(m\) and determine its nature.
Solution
(i) To find stationary points, we set the derivative equal to zero. The derivative is \(\frac{dy}{dx} = 3x^2 - 4x + 5\). The discriminant of this quadratic is \(b^2 - 4ac = (-4)^2 - 4 \times 3 \times 5 = 16 - 60 = -44\), which is negative. Therefore, there are no real roots, and the curve has no stationary points.
(ii) The gradient \(m = 3x^2 - 4x + 5\). To find the stationary value of \(m\), we differentiate \(m\) with respect to \(x\): \(\frac{dm}{dx} = 6x - 4\). Setting \(\frac{dm}{dx} = 0\) gives \(6x - 4 = 0\), so \(x = \frac{2}{3}\). Substituting \(x = \frac{2}{3}\) into \(m = 3x^2 - 4x + 5\), we get \(m = 3 \left( \frac{2}{3} \right)^2 - 4 \left( \frac{2}{3} \right) + 5 = \frac{11}{3}\).
To determine the nature, we find \(\frac{d^2m}{dx^2} = 6\), which is positive, indicating a minimum.