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 (1−x)−3 . I have a solution where I consider :
(1−x)−3=a0+a1x+a2x2+...+anxn+...
Then I multiply it with the following :
(1−x)−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:n∑k=0(k+1)(k+2)2=12n∑k=0k2+32n∑k=0k+n∑k=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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