1→3 2→6 3→10 4→15 5→21 … … The law of finding formula

It can be found that
3=1+2
6=1+2+3
10=1+2+3+4
15=1+2+3+4+5
21=1+2+3+4+5+6
The nth formula is n → (n + 1) (n + 2) / 2
The answer is over

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