If n = 5 then set {n} = {1,2,3,4,5}. When r = 3 ,i have to compute
sum = 1.2.3 + 1.2.4 + 1.2.5 + 1.3.4 + 1.3.5 + 1.4.5 + 2.3.4 + 2.3.5 + 2.4.5 + 3.4.5 [(.) = (*) multiply sign here].
Is there any closed form formula for arbitrary given (N and R)? plz help with this problem...
Thanks in advance...