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Nov 2010 p13 q11
1301
The diagram shows parts of the curves \(y = 9 - x^3\) and \(y = \frac{8}{x^3}\) and their points of intersection \(P\) and \(Q\). The \(x\)-coordinates of \(P\) and \(Q\) are \(a\) and \(b\) respectively.
(i) Show that \(x = a\) and \(x = b\) are roots of the equation \(x^6 - 9x^3 + 8 = 0\). Solve this equation and hence state the value of \(a\) and the value of \(b\).
(ii) Find the area of the shaded region between the two curves.
(iii) The tangents to the two curves at \(x = c\) (where \(a < c < b\)) are parallel to each other. Find the value of \(c\).
Solution
(i) To find the points of intersection, set the equations equal: \(9 - x^3 = \frac{8}{x^3}\). Multiply through by \(x^3\) to get \(9x^3 - x^6 = 8\), which simplifies to \(x^6 - 9x^3 + 8 = 0\). Factor this as \((X - 1)(X - 8) = 0\) where \(X = x^3\). Thus, \(x^3 = 1\) or \(x^3 = 8\), giving \(x = 1\) or \(x = 2\). Therefore, \(a = 1\) and \(b = 2\).
(ii) The area between the curves is given by the integral \(\int_{1}^{2} \left( 9 - x^3 - \frac{8}{x^3} \right) \, dx\). Integrate to find \(\left[ 9x - \frac{x^4}{4} + \frac{4}{x^2} \right]_{1}^{2}\). Evaluating this gives \(18 - 4 + 1 - (9 - \frac{1}{4} + 4) = 2\frac{1}{4}\).
(iii) The derivatives are \(\frac{dy}{dx} = -3x^2\) for \(y = 9 - x^3\) and \(\frac{dy}{dx} = -\frac{24}{x^4}\) for \(y = \frac{8}{x^3}\). Set \(-3c^2 = -\frac{24}{c^4}\) to find \(c\). Solving gives \(c^6 = 8\), so \(c = \sqrt[6]{8}\) or approximately \(1.41 \ldots\).