Question

# Consider that the 1st triangular number is 1, 2nd triangular number is 3, 3rd triangular number is 6 and so on. The 26th triangular number is 351. Determine the 25th triangular number.

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Solution

## A nth triangular number is essentially the sum of the first n natural numbers. 1=1 3=1+2 6=1+2+3 . . . and, so on The 26th triangular number can be written as 1+2+3+..........+24+25+26=351 The sum of the first 25 numbers is the 25th triangular number, so 25th triangular number +26=351 Hence, 25th triangular number =325

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