# Difficulty with permutation and combination

by Taylor_1989
Tags: combination, difficulty, permutation
 P: 106 Right I am having an issue with the proof to permutation, I really can see the $n-r-1$ I think the confusion stems because it is in the general term, which throws me a bit, if possible could someone maybe write it in numbers and the underneath write in the general term if not too much trouble. The reason I ask for this is I am trying to understand the binomial expansion, and I have never done permutation or comnations be for, I do understand factorials and how permutation work and combination, but can't get my head around the proof for the formula. I would like to thank anyone in advance for posting a replie to this post, much appreciated.
 P: 106 Okay so I have resized I am being quite vague, when I posted this. So I have been having a look at the proof a going over it so please correct me if I am wrong to what I am about to write: if I have say 5 letter - ABCDE and have 5 place to fill this I would get this type of equation: $5*4*3*2*1= 120$ So I have 120 permutation. In general form this would N=5 and the R=5 $n*n-1*n-2*n-r+1= 120$. So I am under the assumption that the $n-r+1$ is the last term in the sequence so to speak, correct or not?