2016 is a triangular number.
The first three triangular numbers are: 1, 3, 6. The first three pentagonal numbers are: 1, 5, 12.
The pentagonal numbers 1, 5 and 12 are all one third of a triangular number.
Are all pentagonal numbers one third of a triangular number?
Reveal Solution
Algebra solution
The formula of the th pentagonal number:
i.e. the th triangular number.