题目
Problem
The first term of an arithmetic progression is and the common difference is .
(a) Find the sum of the first terms of the progression.
[2]
It is given that the sum of the first terms is times the sum of the first terms.
(b) Find the value of .
[3]
题目中文翻译
一个等差数列的首项为 ，公差为 。
(a) 求该数列前 项的和。
已知前 项的和是前 项和的 倍。
(b) 求 的值。