Find pair of n numbers in php

Recently I was working on mobile project in which we were working on service code in php, To find the pair in certain number in list. 

Example: 













$total = 4;
$pair_total = $total * ($total - 1) * 0.5; // 6


So here we are able to find the pair combination using this formula.And pls comment if there any tricks similar to this.




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