排列组合问题答:150个.(1)解法一.先选取最后两位数,这样的数有1+2+3+4+5=15种选择(01,02,12,03,13,23,04,14,24,34,05,15,25,35,45),允许首位为零,首位有4种选择,第二位有3种选择,共计15*4*3=180种选择.下面将首位为零且满足最后一位大于前一位的数统计出来,先选取最后两位数,这样的数有1+2...
在1~9中选择4个数进行组合,有哪几种?答:共有126种组合,用树状图来列,不容易出错!所有的组合如下:1234,1235,1236,1237,1238,1239,1245,1246,1247,1248,1249,1256,1257,1258,1259,1267,1268,1269,1278,1279,1289,1345,1346,1347,1348,1349,1356,1357,1358,1359,1367,1368,1369,1378,1379,1389,1456,1457,1458,1459,1467,1468,1469,1...