下面各题怎样简便就怎样算. 101×-×21.
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解:(1)70×(+-),
=70×+70×-70×,
=30+28-8,
=58-8,
=50;
(2)×+×,
=×+×,
=×(+),
=×,
=;
(3)6.4÷50-5.9×,
=6.4×-5.9×,
=(6.4-5.9)×,
=0.5×,
=;
(4)101×-×21,
=(101-21)×,
=80×,
=16;
(5)(+1.6×)×0.25,
=(0.6+0.8)×0.25,
=(1+0.4)×0.25,
=1×0.25+0.4×0.25,
=0.25+0.1,
=0.35;
(6),
=[-(0.5+0.2)]÷,
=[-0.7]÷,
=0÷,
=0.
解析分析:(1)(4)运用乘法分配律简算;
(2)先根据积不变的规律变形,再运用乘法分配律简算;
(3)先把除法变成乘法,再运用乘法分配律简算;
(5)先算小括号里面的乘法,再运用乘法分配律简算;
(6)先算下课后两年的乘法,再算小括号里面的加法,然后算中括号里面的减法,最后算括号外的除法.
点评:乘法分配律是常用的简便运算的方法,要熟练掌握,灵活运用.