A sequence \(\{u_n\}\) which satisfies the condition
\[u_n - u_{n-1} = d, \quad n = 2, 3, 4, \dots,\]
where&nbsp;\(d\) is constant, is called an arithmetic sequence or arithmetic progression (A.P., for short) We adopt the notation \(u_1 = a\), and called constant \(d\) the common difference.

A sequence \(\{u_n\}\) which satisfies the condition

\[u_n - u_{n-1} = d, \quad n = 2, 3, 4, \dots,\]

where \(d\) is constant, is called an *arithmetic sequence* or *arithmetic progression* (A.P., for short) We adopt the notation \(u_1 = a\), and called constant \(d\) the *common difference*.

Definition of AP

Definition of AP