Question

convert n(n-1)(n-2)......(n-r+1) into factorial format.

Answer 1

multiply both numerator and denominator with (n-r+2).....2*1
then it will look like
n(n-1)(n-2)....(n-r+2)(n-r+3)(n-r+4). ..3*2*1/(n-r+2)(n-r+3)..... 3*2*1
n!/(n-r+2)!