Find the sum of first positive 1000 integers.

The sum of first six terms of an arithmetic progression is 42. The ratio of the 10th term to the 30th term is 1 : 3. Calculate the first and the thirteenth term.

In an A.P., the sum of its first n terms is 6n - n². Find its 25th term.

Find the sum of first 15 multiples of 8.