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June 2002 p3 q10
1778
The function \(f\) is defined by \(f(x) = (\ln x)^2\) for \(x > 0\). The diagram shows a sketch of the graph of \(y = f(x)\). The minimum point of the graph is \(A\). The point \(B\) has \(x\)-coordinate \(e\).
(i) State the \(x\)-coordinate of \(A\).
(ii) Show that \(f''(x) = 0\) at \(B\).
(iii) Use the substitution \(x = e^u\) to show that the area of the region bounded by the \(x\)-axis, the line \(x = e\), and the part of the curve between \(A\) and \(B\) is given by \(\int_0^1 u^2 e^u \, du\).
(iv) Hence, or otherwise, find the exact value of this area.
Solution
(i) The minimum point \(A\) occurs where \(x = 1\) because \((\ln x)^2\) is minimized when \(\ln x = 0\), i.e., \(x = 1\).
(iii) The area is given by \(\int_1^e (\ln x)^2 \, dx\). Using the substitution \(x = e^u\), \(dx = e^u \, du\), and the limits change from \(x = 1\) to \(u = 0\) and \(x = e\) to \(u = 1\). The integral becomes \(\int_0^1 u^2 e^u \, du\).
(iv) Evaluate \(\int_0^1 u^2 e^u \, du\) using integration by parts. Let \(v = u^2\) and \(dw = e^u \, du\), then \(dv = 2u \, du\) and \(w = e^u\). The integral becomes \(\left[ u^2 e^u \right]_0^1 - \int_0^1 2u e^u \, du\). Apply integration by parts again to \(\int_0^1 2u e^u \, du\) with \(v = u\) and \(dw = 2e^u \, du\). The result is \(e - 2\).