MA_598R_pweigel_Summer_2009_Lecture_4

$ \text{4.6) Prove that the sum} $

$ \sum_{n=0}^{\infty}{\int_{0}^{\pi/2}{\left(1-\sqrt{\sin x}\right)^n\cos x }dx } $

$ \text{converges to a finite limit, and find its value.} $

$ \text{Solution: } $

$ \sum_{n=0}^{\infty}{\int_{0}^{\pi/2}{\left(1-\sqrt{\sin x}\right)^n\cos x }dx } $

$ = \lim_{ N \rightarrow\infty}{\sum_{n=0}^{N}{\int_{0}^{\pi/2}{\left(1-\sqrt{\sin x}\right)^n\cos x }dx }} \text{ by definition} $

$ = \lim_{ N \rightarrow\infty}{\int_{0}^{\pi/2}{\cos x\sum_{n=0}^{N}{\left(1-\sqrt{\sin x}\right)^n } }dx} \text{ because the sum is finite} $

$ = \int_{0}^{\pi/2}{\cos x\sum_{n=0}^{\infty}{\left(1-\sqrt{\sin x}\right)^n } dx} \text{ by the Monotone Convergence Theorem} $

$ \left(1-\sqrt{\sin x}\right)^n <1 \text{ on } \left(0,\pi/2\right] \text{ so } \cos x \sum_{n=0}^{\infty}{\left(1-\sqrt{\sin x}\right)^n} = \frac{\cos x}{\sqrt{\sin x}} \text{ is finite a.e. on a bounded domain, so the integral exists} $

$ \int_{0}^{\pi/2}{\frac{\cos x}{\sqrt{\sin x}} } dx = \int_{0}^{1}{\frac{1}{\sqrt{x}} } dx = 2 $

-Ben Bartle

Alumni Liaison

EISL lab graduate

Mu Qiao