find the nth pentagonal number in swift

main.swift
func pentagonalNumber(_ n: Int) -> Int {
    // P_n = n(3n - 1) / 2
    return n * (3 * n - 1) / 2
}
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The pentagonalNumber function takes an integer n as input and calculates the nth pentagonal number using the formula P_n = n(3n - 1) / 2. The result is then returned as an integer.

For example, to find the 7th pentagonal number, we can call the function like this:

main.swift
let result = pentagonalNumber(7)
print(result) // Output: 92
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The 7th pentagonal number is 92.