什么是树形图?我们先来看一个问题: [例1]先后抛掷3枚均匀硬币,求出现“2个正面、1个反面”和“1个正面、2个反面”的概率。抛掷硬币,可能出现正面和反面两种结果,硬币均匀,则正、反出现的可能性相同。因此,每掷1枚,都可以用图表示为,树枝状的线段“<”表示有2种等可能的结果出现,先后抛3枚,一个试验是由3个步骤完成的,我们依 What is a tree diagram? Let’s first look at a problem: [Example 1] Three equal coins were thrown successively to find the probability of “2 positives, 1 negatives” and “1 positive and 2 negatives”. When tossing a coin, there may be two positive and negative results. If the coin is even, the possibility of positive and negative appears the same. Therefore, for each toss, it can be represented by a graph. The dendrite segment “<” indicates that there are 2 possible outcomes, such as throwing 3 pieces. One trial is completed in 3 steps.