1投“信”模型辨析在解决计数问题时常常会遇到“寄信”问题,而学生又往往容易“寄”错,下面笔者对此类问题进行辨析归类,以廓清思维。例题:有4封不同的信寄出去,有3个不同的信箱可以投放,有多少种不同的投放方式?错解1:考虑信箱,每个信箱可以接收4封信中的每一封,有4种接收方法,有3个信箱,根据分步计数原理,所以3个信箱接收信的方法共有4~3=64种不同的方法。错解2:考虑信箱,每个信箱可以接收4封信中的 1 vote “letter ” model analysis in solving the problem of counting often encounter “send letters ”, and students tend to “send ” wrong, the author below the classification of such issues to Clear thinking. Example: There are 4 different letters sent out, there are 3 different mailboxes can be put, how many different delivery methods? Wrong 1: consider the mailbox, each mailbox can receive four letters in each of the letters, there Four kinds of receiving methods, there are three mailboxes, according to the principle of step-by-step counting, so there are 4 ~ 3 = 64 different ways to receive letter from 3 mailboxes. Wrong solution 2: consider the mailbox, each mailbox can receive 4 letters