0.917×2013+142×2.013-5.9×20.13 =0.917×2013+0.142×2013-0.059×2013 =(0.917+0.142-0.059)×2013 =2013 36×4.7+15.9×12 =36×4.7+5.3×36 =(4.7+5.3)×36 =360 7.816×1.45+3.14×2.184+1.69×7.816 =7.816×(1.45+1.69)+3.14×2.184 =7.816×3.14+3.14×2.184 =3.14×(7.816+2.184) =31.4 0.9999×0.4+0.1111×6.4 =9×0.1111×0.4+0.1111×6.4 =0.1111×(3.6+6.4) =1.111 点评:巧算中,动手试试是非常必要的,聪明的孩子都希望一下子看透一个题目,一旦与自己想要的有出入就不愿意动手试算,这需要在学习中避免。第一题三个加数中,只能有两个有公共项,但我们对有公共项的试算后,发现又出现了公共项。第二题中出现的9999与1111是两个规则数,需要先尝试。 19.8+199.8+1999.8+19999.8+199999.8 =20+200+2000+20000+200000-0.2×5 =222220-1 =222219 99.9×99.9+9.99 =99.9×99.9+99.9×0.1 =99.9×(99.9+0.1) =9990 |
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