I was given a task to prove:
1(x+1)(x+2)…(x+n)=1(n−1)!n∑i=1(n−1i−1)(−1)i−1x+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)=n∑k=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}
No comments:
Post a Comment