Wednesday, June 7, 2017

binomial coefficients - Proof by induction, or without it if possible?



I was given a task to prove:
1(x+1)(x+2)(x+n)=1(n1)!ni=1(n1i1)(1)i1x+i
I am almost 100% sure this is best solved by induction but to be honest, I'm scared to start using induction for such an expression on a test where I am limited with time. This reminds me of partial fraction decomposition but that binomial coefficient is disturbing me. I'm not sure how difficult this is to prove, but what I am looking for is a way to do this without induction but it seems impossible to do so without it. Induction in general bothers me. I agree it's a fine way of proving something but I would always chose another way around. Unfortunately, most problems I encountered can't be solved any other way. So before you hit the "report as not real question" button, I would like to ask for a HINT on how to solve this, with or without induction?


Answer



Since every k,k=n,,1 is a simple pole of the given fraction then its decomposition take the form



1(x+1)(x+2)...(x+n)=nk=1akx+k
and we have
ak=lim
so yes it's true that

\frac{1}{x(x+1)(x+2)...(x+n)}=\frac{1}{(n-1)!}\sum_{k=1}^n {n-1\choose k-1}\frac{(-1)^{(k-1)}}{x+k}


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