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小学二年级上册奥数知识点专家讲座第5课-3《自然数列趣题》练习及答案

时间:2022-04-23 12:11:19 热传 我要投稿

习题五解答  1.解:分类计算,并将有数字“1”的数枚举出来.  “1”出现在个位上的数有:   1,11,21,31,41,51,61,71,81,91,  101,111,121,131,141,151,161,171,181,191  共20个;  “1”出现在十位上的数有:  10,11,12,13,14,15,16,17,18,19  110,111,112,113,114,115,116,117,118,119  共20个;  “1”出现在百位上的数有:   100,101,102,103,104,105,106,107,108,109,  110,111,112,113,114,115,116,117,118,119,  120,121,122,123,124,125,126,127,128,129,  130,131,132,133,134,135,136,137,138,139,  140,141,142,143,144,145,146,147,148,149,  150,151,152,153,154,155,156,157,158,159,  160,161,162,163,164,165,166,167,168,169,  170,171,172,173,174,175,176,177,178,179,  180,181,182,183,184,185,186,187,188,189,  190,191,192,193,194,195,196,197,198,199  共100个;  数字“1”在1至200中出现的总次数是:  20+20+100=140(次).  2.解:采用枚举法,并分类计算:   “3”在个位上:3,13,23,33,43,53,63,73,83,93共10个;  “3”在十位上:31,33,35,37,39共5个;  数字“3”在1至100的奇数中出现的总次数:  10+5=15(次).  3.解:枚举法:12,17,22,27,32,37,42,47,52,57,62,67,72,77,82,87,92,97共18个.  4.解:分段统计,再总计.  页数 铅字个数  1~9共9页 1×9=9(个)(每个页码用1个铅字)  10~90共90页 2×90=180(个)(每个页码用2个铅字)  100~199共100页 3×100=300(个)(每个页码用3个铅字)  第200页共1页 3×1=3(个)(这页用3个铅字)  总数:9+180+300+3=492(个).  5.解:列表枚举,分类统计:  10 1个  20 21 2个  30 31 32 3个  40 41 42 43 4个  50 51 52 53 54 5个  60 61 62 63 64 65 6个  70 71 72 73 74 75 76 7个  80 81 82 83 84 85 86 87 8个  90 91 92 93 94 95 96 97 98 9个  总数1+2+3+4+5+6+7+8+9=45(个).  6.解:枚举法,再总计:  101,111,121,131,141,151,161,171,181,191共10个.

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