题目
Problem
The first, second and third terms of a progression are , and respectively.
(a) Given that the progression is arithmetic, find the th term.
[2]
(b) Given instead that the progression is geometric, find the sum to infinity.
[4]
题目中文翻译
一个数列的第一项、第二项和第三项分别为 、 和 。
(a) 已知这个数列是等差数列，求第 项。
(b) 若改为已知这个数列是等比数列，求无穷和。