（1）We will call a sequence an arithmetic sequence if there is a common difference
意思是我們稱一個(gè)有相等差別的數(shù)列為算術(shù)數(shù)列,其實(shí)說白了就是我們的等差數(shù)列
如1,3,5,7,9.
（2）A Geometric Sequence is a sequence which the ratio of the common terms is equal.
The general term is
an = a1 * r^（n-1）
意思是:"幾何數(shù)列就是數(shù)字間是同等比例的順序"
說白了就是我們常說的等比數(shù)列
如：3,6,12,24,48...
就是3*2^（n-1）
參考資料里非常詳細(xì),不過是英文版的,有不明白的請(qǐng)來信息