Triplet (x, y, z) is chosen from the set {1, 2, 3,, …., n}, such that x≤y<z .The number of such triplets is
n3
n+1C3
nC2
nC2+nC3
Number of selections when x <y <z =nC3
Number of selections when x = y <z =nC2
(we have to select only two numbers out of n numbers)Required number =nC3+nC2=n+1C3