Q:

Arithmethic Progression problem!

Given that the first term of an arithmetic progression is $5$ and the sum of the first eight terms is equal to the sum of the following four terms, find the common difference. 
What do they mean by " sum of the first eight terms is equal to the sum of the following four terms " ?
Can you show the equation??
Is it something like $S_8=S_{12}-S_4$ ? I am stuck here
Sorry my english is terrible

A:

It means $\sum\limits_{i = 1}^8 a_i = \sum\limits_{i = 9}^{12}a_i$, where $a_i$ is $i$th term given by $a_i = a_1 + (i-1)d$, and $a_1 = 5$.

