a) [tex] \frac{(n+1)!}{n!} = \frac{n!(n+1)}{n!} = n+1[/tex]
b) [tex] \frac{(n+3)!}{n!(n+3)} = \frac{n!(n+1)(n+2)(n+3)}{n!(n+3)} = (n+2)(n+3) = n^{2}+5n+6[/tex]
c) [tex] \frac{(n+1)!n!}{(n-1)!(n+2)!} = \frac{(n-1)!n(n+1)n!}{(n-1)!n!(n+1)(n+2)}= \frac{n}{n+2} [/tex]