Browsing as Guest. Progress, bookmarks and attempts are disabled.
Log in to track your work.
June 2014 p31 q8
1887
(i) By sketching each of the graphs \(y = \csc x\) and \(y = x(\pi - x)\) for \(0 < x < \pi\), show that the equation \(\csc x = x(\pi - x)\) has exactly two real roots in the interval \(0 < x < \pi\).
(ii) Show that the equation \(\csc x = x(\pi - x)\) can be written in the form \(x = \frac{1 + x^2 \sin x}{\pi \sin x}\).
(iii) The two real roots of the equation \(\csc x = x(\pi - x)\) in the interval \(0 < x < \pi\) are denoted by \(\alpha\) and \(\beta\), where \(\alpha < \beta\).
(a) Use the iterative formula \(x_{n+1} = \frac{1 + x_n^2 \sin x_n}{\pi \sin x_n}\) to find \(\alpha\) correct to 2 decimal places. Give the result of each iteration to 4 decimal places.
(b) Deduce the value of \(\beta\) correct to 2 decimal places.
Solution
(i) Sketch the graphs of \(y = \csc x\) and \(y = x(\pi - x)\) for \(0 < x < \pi\). The graph of \(y = \csc x\) has vertical asymptotes at \(x = 0\) and \(x = \pi\), and the graph of \(y = x(\pi - x)\) is a downward-opening parabola with roots at \(x = 0\) and \(x = \pi\). The intersection points of these graphs represent the roots of the equation \(\csc x = x(\pi - x)\). By analyzing the graphs, we find exactly two intersection points in the interval \(0 < x < \pi\).
(ii) Start with \(\csc x = x(\pi - x)\), which is \(\frac{1}{\sin x} = x(\pi - x)\). Rearrange to get \(x = \frac{1 + x^2 \sin x}{\pi \sin x}\).
(iii)(a) Use the iterative formula \(x_{n+1} = \frac{1 + x_n^2 \sin x_n}{\pi \sin x_n}\) starting with an initial guess. Iterate until the value stabilizes to 4 decimal places. The iterations converge to \(\alpha = 0.66\).