1 + 2 ! 3 + 3 ! 6 + 4 ! 1 0 + ⋯ = ?
Notation:
!
is the
factorial
notation. For example,
8
!
=
1
×
2
×
3
×
⋯
×
8
.
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I fail to understand the step of : 1/2 * Σ (n-1)/(n-1)! = 1/2 * Σ n/n!
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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 .
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The n th term of the given series is a n = n ! 2 n ( n + 1 ) = 2 ( n − 1 ) ! n + 1 .
Since n → ∞ lim a n a n + 1 = 0 , by the ratio test this series does converge. We then have that
n = 1 ∑ ∞ a n = n = 1 ∑ ∞ 2 ( n − 1 ) ! n − 1 + 2 = 2 1 n = 1 ∑ ∞ ( n − 1 ) ! n − 1 + n = 1 ∑ ∞ ( n − 1 ) ! 1 = 2 1 n = 1 ∑ ∞ n ! n + n = 0 ∑ ∞ n ! 1 =
2 1 n = 1 ∑ ∞ ( n − 1 ) ! 1 + e = 2 1 n = 0 ∑ ∞ n ! 1 + e = 2 e + e = 2 3 e .