First slide
Combinations
Question

Triplet (x, y, z) is chosen from the set {1, 2, 3,, …., n}, such that xy<z .The number of such triplets is

Moderate
Solution

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

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