The sum of three numbers in A.P. is 15 and the sum of the squares of the extreme terms is 58. Find the numbers.

Find four numbers in A.P. whose sum is 20 and the sum of whose squares is 120.

The angles of a polygon are in A.P. with common difference 5°. If the smallest angle is 120°, find the number of sides of the polygon.

If p^th term of an A.P. is q and its q^th term is p, show that its (p + q)^th term is zero.