項(xiàng)數(shù)公式為：項(xiàng)數(shù)=[(尾數(shù)-首數(shù))/公差]+1。數(shù)列中項(xiàng)的總個(gè)數(shù)為數(shù)列的項(xiàng)數(shù)，項(xiàng)數(shù)是一個(gè)正整數(shù)。無窮數(shù)列沒有項(xiàng)數(shù)。數(shù)列中項(xiàng)的總數(shù)之和為數(shù)列的“項(xiàng)數(shù)”，在數(shù)列中，項(xiàng)數(shù)是一個(gè)正整數(shù)。

項(xiàng)數(shù)在等差數(shù)列中的應(yīng)用：和=（首項(xiàng)+末項(xiàng)）×項(xiàng)數(shù)÷2，項(xiàng)數(shù)=（末項(xiàng)-首項(xiàng)）÷公差+1，首項(xiàng)=2和÷項(xiàng)數(shù)-末項(xiàng)，末項(xiàng)=2和÷項(xiàng)數(shù)-首項(xiàng)(以上2項(xiàng)為第一個(gè)推論的轉(zhuǎn)換)，末項(xiàng)=首項(xiàng)+（項(xiàng)數(shù)-1）×公差。