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# 北师大版数学四年级下册《手拉手》课件

—小数的混合运算、简算

12.8 × 3 = 38.4(元) 7.2 × 3 = 21.6(元) 38.4 + 21.6= 60(元)

12.8×3+7.2×3 = 38.4 + 21.6 = 60(元)

(12.8 + 7.2) ×3 = 20 × 3 = 60(元) 答：一共要花60元。

1、小数混合运算的运算顺序与整数相同。

2、一个算式里，如果机油乘法又有加减法 ，要先算乘法再算加减法，有括号的先算括 号里面的。

12.8 × 3 + 7.2 × 3 = 38.4 + 21.6 = 60(元)

(12.8 + 7.2) × 3 = 20 × 3 = 60(元)

（2）加法结合律：（a＋b）＋c＝ a＋（b＋c ）
（3）乘法交换律： a × b＝b × a （4）乘法结合律：（a×b）×c＝ a×（b×c ）

（5）乘法分配律：（a＋b）×c＝ a×c＋b×c

= 2.6 + 4.2 = 6.8

= 1.5 + 0.9 = 2.4

= 8×1.25+0.8×1.25 = 10 + 1 = 11

= (2.5 × 4) × 2.8 = 10 × 2.8 = 28

= 32.5 + 0.9 = 33.4 = 0.25×0.4×13.7 = 0.1 × 13.7 = 1.37
= 1.83+0.17+3.79 = 2 + 3.79 = 5.79

= 22 – 11.2 = 10.8

= 0.89×(4.8+5.2 ) = 0.89 × 10 = 8.9
= 10–(0.34+0.66) = 10 – 1 =9

500 × 365

= 182500(双)

2.这些一次性筷子大约需要消耗多少棵大树的木材？

182500 ÷ 5000 =

36.5(棵)

0.1 × 36.5

= 3.65(千克) = 27.375(千克)

0.75 × 36.5

0.8 × 1.5

= 1.5(千克)

1.5 ×

=

(千克)

365.25 × 4

= 1461(天)

0.89×4.8+0.89×5.2 10-0.34-0.66

0.25×13.7×0.4

3.2+1.3×4 2.5×2.8×4

（4+0.4）×2.5 22-1.6×7

13+87×0.5 =100×0.5 =50 0.4×8.7×2.5 =(0.4×2.5)×8.7 =10×8.7 =87

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