用简便方法计算(要写出主要的简算过程) (1)(2)(687+687)×50(3)12.5×3.1×80(4).
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解:(1)2×3.4+2.5×6,
=2.5×3.4+2.5×6.6,
=2.5×(3.4+6.6),
=2.5×10,
=25;
(2)(687+687)×50,
=687×2×50,
=687×(2×50),
=687×100,
=68700;
(3)12.5×3.1×80,
=12.5×80×3.1,
=1000×3.1,
=3100;
(4)(++)÷,
=(++)×12,
=×12+×12+×12,
=3+4+2,
=9.
解析分析:(1)把分数化为小数,运用乘法分配律简算;
(2)括号内为687×2,运用乘法结合律简算;
(3)运用乘法交换律简算;
(4)把除法改为乘法,运用乘法分配律简算.
点评:此题是考查四则混合运算,要仔细观察算式的特点,灵活运用一些定律进行简便计算.