Triangle numbers or triangular numbers are those numbers that can form an equilateral triangle when counting the objects. The first five triangular numbers are: 1, 3, 6, 10, 15.
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The first six triangular numbers are : 1,3,6,10,15,21. However, sometimes the first triangular number is regarded as 0 (zero) so you then have 0,1,3,6,10,15 as the first six triangular numbers.
8,9,5,7,2 and 1
It is 46.
1+1+1+1+1+=5 * * * * * The question did not ask for the sum of the first counting number five times! The sum of the first 5 counting numbers is 1+2+3+4+5 = 15. Such sums are known as triangular numbers.
Oh, dude, triangular numbers are like the cool kids of math, they're all about those equilateral vibes. So, between 11 and 39, we've got 15 and 21 as triangular numbers. That's it, just those two. It's like a tiny exclusive club, you know?