There is a stair of 10 steps.IF you can only go up for 1 ste

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There is a stair of 10 steps.IF you can only go up for 1 steps or 2 steps once.How many ways can
you use to get to the 10th step?

10级台阶可以分成:
10个1级;
8个1级,1个2级;
6个1级,2个2级;
4个1级,3个2级;
2个1级,4个2级;
5个2级
共6种分台阶的办法,
然后排列组合,
共有,1+9C1+(7C2+7C1)+(5C3+5C2+5C1)+(5C2+5C1)+1=79 种上楼的方法
再问：不对把，我看了答案是89，就是不知道为什么
再答：确定吗？答案有解析式子吗？我和我朋友都算的79种
再问：没，你会不会漏了
再答：哦，我想明白了，对不起对不起，之前算错了，在“4个1级，3个2级”里面少算10种，应该是 1+9C1+(7C2+7C1)+(5C3+(5C2)*2+5C1)+(5C2+5C1)+1=89 种上楼的方法复制搜索

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