Triple Summation?

Calculus Level 5

lim h ( i = 1 i = h ( ( j = 1 j = i ( 2 j + 1 ) ! k = 1 k = j ( 2 k 1 ) ) + 2 ) ( ( i + 1 ) ! ) 2 ) \displaystyle \LARGE \lim _{ h\rightarrow \infty }{ \left( \sum _{ i=1 }^{ i=h }{ \frac { \left( \left( \sum _{ j=1 }^{ j=i }{ \frac { \left( 2j+1 \right) ! }{ \prod _{ k=1 }^{ k=j }{ \left( 2k-1 \right) } } } \right) +2 \right) }{ { \left( (i+1)! \right)}^{ 2 } } } \right) }

If the answer can be expressed as e a b {e}^{a}-b , select a + b a+b .

Try its sister problem here
5 3 8 4 6 7 2 1

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1 solution

Tanishq Varshney
Nov 13, 2015

k = 1 j ( 2 k 1 ) = ( 2 j 1 ) ! ! \displaystyle \prod _{k=1}^{j} (2k-1)=(2j-1)!!

Also ( 2 j ) ! ( 2 j 1 ) ! ! = 2 j j ! \large{\frac{(2j)!}{(2j-1)!!}=2^{j}j!}

Now we get

j = 1 i ( 2 j + 1 ) 2 j j ! \large{\displaystyle \sum^{i}_{j=1} (2j+1)2^{j}j!}

j = 1 i 2 j + 1 ( j + 1 1 ) j ! + 2 j j ! \large{\displaystyle \sum^{i}_{j=1} 2^{j+1}(j+1-1)j! +2^j j! }

j = 1 i 2 j + 1 ( j + 1 ) ! 2 j j ! \large{\displaystyle \sum^{i}_{j=1} 2^{j+1} (j+1)!-2^j j!}

2 i + 1 ( i + 1 ) ! 2 \rightarrow 2^{i+1}(i+1)!-2

Finally we have

lim h i = 1 h 2 i + 1 ( i + 1 ) ! 2 + 2 ( ( i + 1 ) ! ) 2 \large{\displaystyle \lim_{ h \to \infty}\displaystyle \sum^{h}_{i=1} \frac{2^{i+1}(i+1)!-2+2}{((i+1)!)^2}}

lim h i = 1 h 2 i + 1 ( i + 1 ) ! \large{\displaystyle \lim_{ h \to \infty}\displaystyle \sum^{h}_{i=1} \frac{2^{i+1}}{(i+1)!}}

k = 2 2 k k ! k = 0 2 k k ! 1 0 ! 2 1 ! \large{\displaystyle \sum_{k=2}^{\infty} \frac{2^k}{k!} \rightarrow \displaystyle \sum_{k=0}^{\infty} \frac{2^k}{k!}-\frac{1}{0!}-\frac{2}{1!} }

We know that n = 0 x n n ! = e x \large{\displaystyle \sum_{n=0}^{\infty} \frac{x^n}{n!}=e^x}

Hence

e 2 3 \large{\boxed{e^2-3}}

Solved the same way ! 😀

Anurag Pandey - 4 years, 8 months ago

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