a) Show that the sum of the first n terms of an arithmetic progression with first term "a" and common difference "d" is n/2[2a+(n-1)d].
b) An arithmetic progression with first term 5, and common difference 2, consist of 3n terms. Given that the sum of the first n_term and the sum of the last n_term are on the ratio 7:27,find n and the sum of the middle n_term d. Hence show that the sum of the first n_term and the middle n_terms are in the ratio 7:17​