题目内容
【题目】用递等式计算．（能简算的要简算）
20.9+10.5÷（5.2﹣3.5）
3.5×0.7+5.5×0.7+0.7
10.64+7.65×2.4+11.76
6.2×9.9
0.125×3.2×0.25
9.83×（3.8﹣2.3）+1.5×6.17
21÷0.25÷0.4．
【答案】
①20.9+10.5÷（5.2﹣3.5）
=20.9+10.5÷1.7
=20.9+6
=27
②3.5×0.7+5.5×0.7+0.7
=（3.5+5.5+1）×0.7
=10×0.7
=7
③10.64+7.65×2.4+11.76
=10.64+18.36+11.76
=29+11.76
=40.76
④6.2×9.9
=6.2×（10﹣0.1）
=6.2×10﹣0.1×6.2
=62﹣0.62
=61.38
⑤0.125×3.2×0.25
=0.125×8×0.4×0.25
=（0.125×8）×（0.4×0.25）
=1×0.1
=0.1
⑥9.83×（3.8﹣2.3）+1.5×6.17
=9.83×1.5+1.5×6.17
=（9.83+6.17）×1.5
=16×1.5
=4×（4×1.5）
=4×6
=24
⑦21÷0.25÷0.4
=21÷（0.25×0.4）
=21÷0.1
=210
【解析】
试题分析：①先算小括号里面的算式，再算除法最后算加法．
②用乘法分配律计算即可．
③先算除法算式，再从左往右计算即可．
④把9.9变成10﹣0.1，再运用乘法结合律计算，即6.2×9.9=6.2×（10﹣0.1）=6.2×10﹣0.1×6.2．
⑤把3.2看作8×0.4，再运用乘法结合律计算，即（0.125×8）×（0.25×0.4）．
⑥先算小括号里面的算式，再用乘法分配律计算即可．
⑦用连除的性质计算即可．a÷b÷c=a÷（b×c）．
解：①20.9+10.5÷（5.2﹣3.5）
=20.9+10.5÷1.7
=20.9+6
=27
②3.5×0.7+5.5×0.7+0.7
=（3.5+5.5+1）×0.7
=10×0.7
=7
③10.64+7.65×2.4+11.76
=10.64+18.36+11.76
=29+11.76
=40.76
④6.2×9.9
=6.2×（10﹣0.1）
=6.2×10﹣0.1×6.2
=62﹣0.62
=61.38
⑤0.125×3.2×0.25
=0.125×8×0.4×0.25
=（0.125×8）×（0.4×0.25）
=1×0.1
=0.1
⑥9.83×（3.8﹣2.3）+1.5×6.17
=9.83×1.5+1.5×6.17
=（9.83+6.17）×1.5
=16×1.5
=4×（4×1.5）
=4×6
=24
⑦21÷0.25÷0.4
=21÷（0.25×0.4）
=21÷0.1
=210
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