比一比 看谁算得快0.32×100 = 32 15-0.15 = 14.85 1.25×8 = 10 0.24×5 = 1.2 24+0.24 = 24.24 0.4×0.5 = 0.2 2.5×4 = 10 0.6×0.1 = 0.06

比一比 看谁算得巧

25×73×4= 25×4×73 = 100×73 = 7300

(23×4)×15= 23×(4×15) = 23×60 = 1380

102×32= (100+2)×32 = 100×32+2×32 = 3200+64 = 3264

分组计算男生组 0.8×1.3 (0.9×0.4)×0.5 (3.2+2.8) ×0.6

= = =

女生组 1.3×0.8 0.9×(0.4×0.5) 3.2×0.6+2.8×0.6

观察每组的两个算式有什么不同？ 你能得出什么结论？

结论：整数的运算律同样适用于小数。

练一练7.6×0.8+0.2×7.6 = 7.6×(0.8+0.2) = 7.6×1 = 7.6 76×0.08+0.2×7.6 = 7.6×0.8+0.2×7.6 = 7.6×(0.8+0.2) = 7.6×1 = 7.6 0.25×36 = 0.25×4×9 = 1×9 =9

0.85×99 = 0.85×(100-1) = 0.85×100-0.85×1 = 85-0.85 = 84.15

0.25×32×1.25 0.85×99+0.85 =0.25×4×8×1.25 = 0.85×99+0.85×1 = (0.25×4)×(8 ×1.25) = 0.85×(99+1) = 1×10 = 0.85×100 = 10 =85

练一练0.25×8.5×4 0.25×(8.5+8.5+8.5+8.5) = 0.25×(8.5×4) = 0.25×4×8.5 = 1×8.5 =8.5 1.28×8.6+0.72×8.6 1.28×8.6+1.44×4.3 = 1.28×8.6+0.72×2×4.3 = 1.28×8.6+0.72×8.6 = (1.28+0.72) ×8.6 = 2×8.6 =17.2 2.4×1.02 2.4×1.02-0.048 = 2.4×(1+0.02)-0.048 = 2.4×1.02-2.4×0.02 = 2.4×1+2.4×0.02-0.048 = 2.4×(1.02-0.02) = 2.4+0.048-0.048 = 2.4×1 =2.4 = 2.4 1.28×8.6+0.82×8.6-0.86 = 1.28×8.6+0.82×8.6-0.1×8.6 = (1.28+0.82-0.1) ×8.6 = 2×8.6 =17.2

回顾一下，通过本节课的学习，你有 哪些新的收获？小数计算应注意的问题： 一审：审清题目。 二看：观察数字特征，选择合理的运算律。 三算：认真计算。 四查：查运算顺序；查数字；查每一步的计算。

没有大胆的猜测，就做不出伟大的发现。——牛顿

数学中的一些美丽定理具有这样的特性:它们 极易从事实中归纳出来, 但证明却隐藏的深。 ——高斯