等比数列的通项公式及前ｎ项和公式是两个很重要的公式 ,同学们在运用时往往会出现一些错误 ,现总结如下 .1　搞不清“首项”与“项数”导致错误例 1　 (高中数学课本第一册Ｐ1 2 9第三题 )某种细菌在培养过程中 ,每半小时分裂一次 (一个分裂为两个 ) ,经过 4小时 ,这种细菌由 The general formulae of the geometric progression and the first n terms and formulae are two very important formulas. Students often have some mistakes when using them. They are summarized as follows.1 It is not clear why the “first item” and the “number of items” cause confusion. Error Example 1 (High School Mathematics Textbook Volume 1 P1 2 9 Problem 3) In the course of cultivation, a certain bacteria splits every half an hour (one split into two). After 4 hours, the bacteria are