Sumtegrals! - 2

Calculus Level 4

K = n = 0 9 n ( n + 3 ) ( n + 2 ) n ! K = \displaystyle\sum_{n=0}^{\infty} \frac{9^n}{(n+3)(n+2)n!}

If the infinite sum above can be expressed in the form 1 a ( c e b + d ) \frac{1}{a} (ce^b + d) , find the value of a + b + c + d + 1 a + b + c + d + 1


The answer is 757.

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2 solutions

Maggie Miller
Jul 19, 2015

Through partial fraction decomposition and integration, we get

K = n = 0 9 n n ! ( 1 n + 2 1 n + 3 ) = n = 0 9 n n ! ( n + 2 ) n = 0 9 n n ! ( n + 3 ) \displaystyle K=\sum_{n=0}^\infty\frac{9^n}{n!}\left(\frac{1}{n+2}-\frac{1}{n+3}\right)=\sum_{n=0}^\infty\frac{9^n}{n!(n+2)}-\sum_{n=0}^\infty\frac{9^n}{n!(n+3)}

= d d x ( n = 0 x n + 1 ( n + 2 ) ! ) x = 9 d 2 d x 2 ( n = 0 x n + 2 ( n + 3 ) ! ) x = 9 \displaystyle=\frac{d}{dx}\left(\sum_{n=0}^{\infty}\frac{x^{n+1}}{(n+2)!}\right)\bigg|_{x=9}-\frac{d^2}{dx^2}\left(\sum_{n=0}^{\infty}\frac{x^{n+2}}{(n+3)!}\right)\bigg|_{x=9}

= d d x ( 1 x n = 0 x n n ! 1 x 1 ) x = 9 d 2 d x 2 ( 1 x n = 0 x n n ! + 1 x + 1 + x 2 ) x = 9 \displaystyle=\frac{d}{dx}\left(\frac{1}{x}\sum_{n=0}^{\infty}\frac{x^n}{n!}-\frac{1}{x}-1\right)\bigg|_{x=9}-\frac{d^2}{dx^2}\left(\frac{1}{x}\sum_{n=0}^\infty \frac{x^n}{n!}+\frac{1}{x}+1+\frac{x}{2}\right)\bigg|_{x=9}

= d d x ( e x x 1 x 1 ) x = 9 d 2 d x 2 ( e x x 1 x 1 x 2 ) x = 9 \displaystyle=\frac{d}{dx}\left(\frac{e^x}{x}-\frac{1}{x}-1\right)\bigg|_{x=9}-\frac{d^2}{dx^2}\left(\frac{e^x}{x}-\frac{1}{x}-1-\frac{x}{2}\right)\bigg|_{x=9}

= ( e x x 1 x 2 + 1 x 2 ) x = 9 ( e x x 2 e x x 2 + 2 e x x 3 2 x 3 ) x = 9 \displaystyle=\left(\frac{e^x}{x}-\frac{1}{x^2}+\frac{1}{x^2}\right)\bigg|_{x=9}-\left(\frac{e^x}{x}-\frac{2e^x}{x^2}+\frac{2e^x}{x^3}-\frac{2}{x^3}\right)\bigg|_{x=9}

= 1 81 ( 1 + 8 e 9 ) + 1 729 ( 2 65 e 9 ) = 1 729 ( 7 e 9 + 11 ) . \displaystyle=\frac{1}{81}(1+8e^9)+\frac{1}{729}(2-65e^9)=\frac{1}{729}(7e^9+11).

Therefore, the answer is 729 + 9 + 7 + 11 + 1 = 757 729+9+7+11+1=\boxed{757} .

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