3rdoctagonalnumber

<-- Enter octagonal number statement
  

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Calculate octagonal number 3

The formula for octagonal number n is as follows
O_n = n(3n - 2)

Given n = 3, we have:

O_n = 3(3(3) - 2)
O_n = 3(9 - 2)
O_n = 3(7)
O₃ = 21

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