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Unformatted text preview: MATH 138 Assignment 7 Solutions Problem 1: Find the sums of the following series. (a) X n =0 n 2 3 n 1 Solution: X n =0 n 2 3 n 1 = X n =0 2 n (2 3 ) n = X n =0 2 8 n Note: Geometric Series with a = 2,  r  < 1 = 2 1 8 = 16 8 (b) X n =0 3 + 2 n 3 n +2 Solution: X n =0 3 + 2 n 3 n +2 = X n =0 3 3 n +2 + X n =0 2 n 3 n +2 X n =0 3 3 n +2 = X n =0 1 3 1 3 n Note: Geometric Series with a = 1 3 ,  r  < 1 = 1 3 1 1 3 = 1 3 2 3 = 1 2 X n =0 2 n 3 n +2 = X n =0 1 9 2 3 n Note: Geometric Series with a = 1 9 ,  r  < 1 = 1 9 1 2 3 = 1 9 1 3 = 1 3 X n =0 3 + 2 n 3 n +2 = 1 2 + 1 3 = 5 6 Problem 2a: Use the integral test to show that the series X n =1 ln n n 2 converges. Solution: Let f ( x ) = ln x x 2 . For x 1, f is continuous and f ( x ) 0. f ( x ) = 1 x 1 x 2 + ln x  2 x 3 = 1 2 ln x x 3 f ( x ) < 0 for x 2. We can apply the integral test for the series X n =2 ln n n 2 . Z 2 ln x x 2 dx Let u = ln x dv = 1 x 2 dx du = 1 x dx v = 1 x = lim t  ln x x t 2 Z t 2 1 x 2 dx = lim t  ln t t + ln 2 2 1 x t 2 = lim t  2 ln t t + ln 2 2 + 1 2 = ln 2 2 + 1 2 + lim t  2 t 1 by LHopitals Rule = ln 2 2 + 1 2 Since R 2 ln x x 2 dx converges, so does X n =2 ln n n 2 . We can then add the case when n = 1. Thus, X n =1 ln n n 2 converges. Problem 2b: If s 50 is the partial sum of the above series and s is the full sum of this series, use the remainder estimate for the integral test to show that 9 / 100 s s 50 10 / 100 . Does this series feel like it is converging fast or slowly? This is a matter of opinion and so there is no wrong answer....
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 Winter '07
 Anoymous
 Calculus, Geometric Series

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