Yes, it converges

Calculus Level 3

1 + 3 2 ! + 6 3 ! + 10 4 ! + = ? \large 1 + \frac{3}{2!} + \frac{6}{3!} + \frac{10}{4!} + \cdots = \,?


Notation: ! ! is the factorial notation. For example, 8 ! = 1 × 2 × 3 × × 8 8! = 1\times2\times3\times\cdots\times8 .

e 3 \frac{e}{3} e 2 2 \frac{e^2}{2} 3 e 2 \frac{3e}{2} e 6 \frac{e}{6}

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1 solution

The n n th term of the given series is a n = n ( n + 1 ) 2 n ! = n + 1 2 ( n 1 ) ! a_{n} = \dfrac{\dfrac{n(n + 1)}{2}}{n!} = \dfrac{n + 1}{2(n - 1)!} .

Since lim n a n + 1 a n = 0 \displaystyle\lim_{n \to \infty} \dfrac{a_{n+1}}{a_{n}} = 0 , by the ratio test this series does converge. We then have that

n = 1 a n = n = 1 n 1 + 2 2 ( n 1 ) ! = 1 2 n = 1 n 1 ( n 1 ) ! + n = 1 1 ( n 1 ) ! = 1 2 n = 1 n n ! + n = 0 1 n ! = \displaystyle\sum_{n=1}^{\infty} a_{n} = \sum_{n=1}^{\infty} \dfrac{n - 1 + 2}{2(n - 1)!} = \dfrac{1}{2}\sum_{n=1}^{\infty} \dfrac{n - 1}{(n - 1)!} + \sum_{n=1}^{\infty} \dfrac{1}{(n - 1)!} = \dfrac{1}{2}\sum_{n=1}^{\infty} \dfrac{n}{n!} + \sum_{n=0}^{\infty} \dfrac{1}{n!} =

1 2 n = 1 1 ( n 1 ) ! + e = 1 2 n = 0 1 n ! + e = e 2 + e = 3 e 2 \displaystyle\dfrac{1}{2}\sum_{n=1}^{\infty} \dfrac{1}{(n - 1)!} + e = \dfrac{1}{2}\sum_{n=0}^{\infty}\dfrac{1}{n!} + e = \dfrac{e}{2} + e = \boxed{\dfrac{3e}{2}} .

I fail to understand the step of : 1/2 * Σ (n-1)/(n-1)! = 1/2 * Σ n/n!

maximos stratis - 4 years, 1 month ago

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The sum on the left starts at 1, but (n - 1)/(n - 1)! = 0/0! = 0/1 = 0 for n = 1 so

n = 1 n 1 ( n 1 ) ! = n = 2 n 1 ( n 1 ) ! = n = 1 n n ! \displaystyle \sum_{n=1}^{\infty} \dfrac{n - 1}{(n - 1)!} = \sum_{n=2}^{\infty} \dfrac{n - 1}{(n - 1)!} = \sum_{n=1}^{\infty} \dfrac{n}{n!} .

Brian Charlesworth - 4 years, 1 month ago

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Ok thank you :)

maximos stratis - 4 years, 1 month ago

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