Saturday, 2 February 2013

sequences and series - Finding Sigmafrac(n+1)(n+2)2! .




How to find the summation of Σ(n+1)(n+2)2! ?





MY WORK:



I know that the expression in the summation is the general term of the binomial expansion (1x)3 . I have a solution where I consider :
(1x)3=a0+a1x+a2x2+...+anxn+...
Then I multiply it with the following :
(1x)1=1+x+x2+x3+...+xn+...
Equating co efficients of xn , I get :
a0+a1+a2+...+an=Σ(n+1)(n+2)2!=(n+1)(n+2)(n+3)3! ...




CAN I DO IT IN ANY OTHER WAY OTHER THAN BINOMIAL EXPANSION?


Answer



You can do it like this:nk=0(k+1)(k+2)2=12nk=0k2+32nk=0k+nk=01=n(n+1)(2n+1)12+3n(n+1)4+n+1.Now, all it takes is to check thatn(n+1)(2n+1)12+3n(n+1)4+n+1=(n+1)(n+2)(n+3)6.


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