Examples of A.P

In the A.P. nth term of an A.P. is given by (-4n + 15). Find the sum of first 20 terms of A.P.

If p ,q ,a n dr are in A.P., then which of the following is/are true? pth, qth, and rth terms of A.P. are in A.P. pth, qth, rth terms of G.P. are in G.P. pth, qth, rth terms of H.P., are in H.P. none of these