Oplease show the works.

Problem 4. A varying immediate annuity with a term of 2n years has a first payment equal to 1. Thereafter, payments increase by 1 each year until they reach n at the end of n years. Payments remain at n for year n + 1, and then decrease by 1 each year, with a final payment of 1 at the end of 2n years. Derive an expression for the present value of this annuity. (A) an ( **) (B) a(2+) (C) a(a - ) (D) am ( + n) (E) a ml (1 + ante