认识三角形 (5).ppt

R四年级下册,数学广角鸡兔同笼,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,笼子里有若干只鸡和兔。从上面数，有8个头，从下面数，有26只脚。鸡和兔各有几只？,探究新知,分析：,8,2,4,枚举法,1.当鸡有8只时，兔子0只，共16只脚；,2.当鸡有7只时，兔子1只，共18只脚；,3.当鸡有6只时，兔子2只，共20只脚；,2,20,3,22,4,4,24,3,5,26,2,6,28,1,7,30,0,8,32,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,笼子里有若干只鸡和兔。从上面数，有35个头，从下面数，有94只脚。鸡和兔各有几只？,思考：这一题用枚举法好不好做？,假设法,笼子里有若干只鸡和兔。从上面数，有35个头，从下面数，有94只脚。鸡和兔各有几只？,352=70,94-70=24,思考：相差的24只脚是怎么产生的？,假设笼子里都是鸡，共有只脚。,笼子里实际有94只脚，相差只脚。,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,假设法,笼子里有若干只鸡和兔。从上面数，有35个头，从下面数，有94只脚。鸡和兔各有几只？,因为笼子还有兔子，现在我们要将笼子里的部分鸡换成兔子，将1只鸡换成1只兔子，增加了只脚；增加24只脚需要换只鸡。,2,242=12,假设法,笼子里有若干只鸡和兔。从上面数，有35个头，从下面数，有94只脚。鸡和兔各有几只？,所以兔子就有只,鸡有只。,242=12,35-12=23,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,方法总结：首先，假设笼中全是鸡或者兔子，求出假设时的脚数，然后把假设时的脚数与实际情况相比较，找到差距和造成差距的原因，最后经过调整找到正确结果。,注意：假设法是设鸡得兔，或设兔得鸡；假设全是鸡，最后求出来的是兔子的数量。,抬腿法,笼子里有若干只鸡和兔。从上面数，有35个头，从下面数，有94只脚。鸡和兔各有几只？,假设笼子中的鸡和兔都训练有素，只要主人一吹口哨，鸡立马抬起一只脚，开始表演金鸡独立，兔子也会立刻抬起两只脚卖萌；,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,吹哨法,笼子里有若干只鸡和兔。从上面数，有35个头，从下面数，有94只脚。鸡和兔各有几只？,942=47,现在地上一共有只脚，,这时，每只鸡1只脚，每只兔子2只脚，也就是说笼子里只要有一只兔子，脚的总数就比头的总数多1；,吹哨法,笼子里有若干只鸡和兔。从上面数，有35个头，从下面数，有94只脚。鸡和兔各有几只？,47-35=12,脚的总数与头的总数之差,兔子有只,鸡有只。,12,35-12=23,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,闯关练习,1.有龟和鹤共40只，龟的腿和鹤的腿共有112条。龟、鹤各有几只？,（1）如果都是鹤。,如果都是鹤，就有40280条腿，比题目中少1128032条腿。那么需要用龟换鹤，换上一只龟，腿的总数就多2条，有32216只龟。所以有401624只鹤。,解答：,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,如果都是龟，就有404160条腿，比题目中多16011248条腿。那么需要用鹤换龟，换上一只鹤，腿的总数就少2条，有48224只鹤。所以有402416只龟。,（2）如果都是龟。,假如让鹤抬起一条腿，龟抬起两条腿，还有112256条腿。这时，只要有一只龟，则腿的总数就比头的总数多1。这时腿的总数与头的总数之差56-4016，就是龟的只数，所以有401624只鹤。,（3）抬腿法。,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,状元成才路,1.有鸡和兔共20只，有56只脚。鸡（）只，兔（）只。2.小方有面值2角和5角的邮票共12枚，面值总额39角。2角的邮票有（）枚，5角的邮票有（）枚。,12,8,7,5,闯关练习,假设全买篮球，426=252（元）252-210=42（元）排球：42(42-28)=3(个)篮球：6-3=3(个),2.,篮球和排球各买了几个？,状元成才