triangles.These are the first few triangular numbers.

1 3 6 10 15

The way to get these numbers without drawing pictures is to add up

all the numbers that come before a certain number. For example,

the tenth triangular number is 1+2+3+4+5+6+7+8+9+10. Another way

to get the nth triangular number (n is any number bigger than

zero) is to multiply n by one more than n and then divide by 2.

This comes out exactly the same. By this method, the tenth

triangular number is 10*11/2 = 55.

so, basically the formula you can use is [n(n+1)] /2

Example: if you are looking for the 5th number in a triangular series you would do the following: 5(6) / 2

so the fifth number is 15.

Hope this helps