The hexagonal numbers are associated with this sequence of d

The hexagonal numbers are associated with this sequence of dot patterns. The first four hexagonal numbers are shown. What is the 15th hexagonal number?

Solution

The sequence for hexagonal numbers is 1, 6, 15,28,45,...

The nth term is n*(2n-1) .

Hence the 15th term is = 15(2*15-1) = 15*29 = 435