题目内容
18.脱式计算:能简算就简算6.42-2.8+3.58
1.26×0.72-0.26×0.72
12÷2.5÷0.4
5.1×[1.8÷(4.8-3.9)]
7.1×(1.2-0.2÷0.8)
[6.5+(2.8-1.3)]×0.4.
分析 (1)根据加法的交换律简算.
(2)用乘法分配律计算.a×c-b×c=(a-b)×c.
(3)用连除的性质计算.a÷b÷c=a÷(b×c).
(4)先算减法,再算除法,最后算乘法.
(5)先算除法,再算减法,最后算乘法.
(6)先算减法,再算加法,最后算乘法.
解答 解:(1)6.42-2.8+3.58
=6.42+3.58-2.8
=10-2.8
=7.2
1.26×0.72-0.26×0.72
=(1.26-0.26)×0.72
=1×0.72
=0.72
12÷2.5÷0.4
=12÷(2.5×0.4)
=12÷1
=12
5.1×[1.8÷(4.8-3.9)]
=5.1×[1.8÷0.9]
=5.1×2
=10.2
7.1×(1.2-0.2÷0.8)
=7.1×(1.2-0.25)
=7.1×0.95
=6.745
[6.5+(2.8-1.3)]×0.4
=[6.5+1.5]×0.4
=8×0.4
=3.2
点评 考查了小数的四则混合运算顺序,有括号的要先算括号里面的,没有括号的要先算乘除法再算加减法.能用简便方法计算的要用简便方法计算.
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