I don't know how to solve 4b II

Let {un }, n E Z , be an arithmetic sequence with first term equal to a and common difference of d. where d # 0. Let another sequence {vn }, n E Z+, be defined by Un = 2the. 4a. (i) Show that it is a constant. [4 marks] (li) Write down the first term of the sequence { Un}- (iii) Write down a formula for Un in terms of a, d and n. 4b. Let S, be the sum of the first n terms of the sequence { un} [8 marks] (1) Find S., in terms of a, d and n. (il) Find the values of d for which i-lv, exists. S You are now told that iml v, does exist and is denoted by Soo. (ili) Write down So. in terms of a and d . (iv) Given that Soo = 2"+1 find the value of d