等差 (比 )数列作为特殊数列具有一些很好的性质，在解题时应注意灵活运用 .　　一、运用通项变形公式 　　在五个基本量 a1， d（ q）， n， an， sn中，可用方程或方程组“知三求二” .但若用下述变形公式，有些问题的解决就变得很简单 .　　对等差数列 {an}，有 an－ am=（ The equal-difference (comparison) sequence has some good properties as a special sequence, and should be used flexibly when solving problems. First, the general-purpose transformation formula is used in the five basic quantities a1, d(q), n, an, sn You can use the equations or equations to “know three, find two.” However, if you use the following deformation formula, some problems will be solved simply. For the arithmetic sequence {an}, there is an-am=(