Re : Calculate the sum of this infinite series

S n = 1 + 1 2 + 1 3 + 1 4 + + 1 n S_n = 1 + \frac 12 + \frac 13 + \frac 14 + \cdots + \frac 1n

What is the value of lim n S n \displaystyle \lim_{n \to \infty}S_n ?

1.8 1.8 \infty 2 2 1.5 1.5

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2 solutions

Mr. India
Mar 31, 2019

1 = 1 1=1

1 2 + \frac{1}{2}+ 1 3 + \frac{1}{3}+ 1 4 = \frac{1}{4}= 1 4 + \frac{1}{4}+ 1 4 + \frac{1}{4}+ 1 3 + \frac{1}{3}+ 1 4 > 1 \frac{1}{4}>1

1 5 + 1 6 + . . . . + 1 25 = 1 25 + 1 25 + 1 25 + 1 25 + 1 25 + 1 6 + . . . + 1 25 > 1 \frac{1}{5}+\frac{1}{6}+....+\frac{1}{25}=\frac{1}{25}+\frac{1}{25}+\frac{1}{25}+\frac{1}{25}+\frac{1}{25}+\frac{1}{6}+...+\frac{1}{25}>1

So, 1 n \frac{1}{n} + 1 n + 1 + . . . 1 n 2 > 1 +\frac{1}{n+1}+...\frac{1}{n^2}>1

So, 1 + 1 2 + 1 3 + . . . . . > 1 + 1 + 1 + 1..... = i n f i n i t e 1+\frac{1}{2}+\frac{1}{3}+.....>1+1+1+1.....=\boxed{infinite}

Srinivasa Gopal
Mar 30, 2019

Let S = (1/2 + 1/3 + 1/4 + 1/5. + 1/6...)
S is greater than (1/2 + 1/4 + 1/4 + 1/5 + 1/6 + 1/7 + 1/8 + 1//9......) , this inturn is greater than
( 1/2 + 1/4 + 1/4 + 1/8+1/8+1/8+1/8 + 1/9...) or (1/2 + 2/4 + 4/8 + 8/16.....) or N/2 when N tends to infinity, S > N/2
So the LHS S is greater than N/2 when N tends to infinity so the LHS also tends to infinity for large values of nor number of terms. This series is actually divergent and the sum is infinity


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