Question

find the sum of arthmetic progression 12,15,18,,,,,, 84,

a=12
d=3
n=?

Answer 1

First you have to find out how many terms there are, so find out which term number 84 is
84 = a + (n-1)d
84 = 12 + (n-1)(3)
72 = 3n - 3
3n = 75
n = 25

so now that we know we have 25 terms, just use the formula for the sum of 25 terms.

Answer 2

For more information in google type "wikipedia arithmetic progression"