小学三年级奥数趣味学习——速算与巧算(加减混合式的巧算)
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在只有加减运算的算式里,如果括号前面是“+”号,则不论去掉括号或添上括号,括号里面的运算符号都不变;如果括号前面是“-”号,则不论去掉括号或添上括号,括号里面的运算符号都要改变,“+”变“-”,“-”变“+”,即:
a+(b+c+d)=a+b+c+d
a-(b+a+d)=a-b-c-d
a-(b-c)=a-b+c
![](http://pic.ikafan.com/imgp/L3Byb3h5L2h0dHBzL2ltYWdlMTA5LjM2MGRvYy5jbi9Eb3dubG9hZEltZy8yMDIxLzAzLzAyMjEvMjE2NzA4MzMwXzJfMjAyMTAzMDIwOTQ1MjMzMzM=.jpg)
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![](http://pic.ikafan.com/imgp/L3Byb3h5L2h0dHBzL2ltYWdlMTA5LjM2MGRvYy5jbi9Eb3dubG9hZEltZy8yMDIxLzAzLzAyMjEvMjE2NzA4MzMwXzRfMjAyMTAzMDIwOTQ1MjM0NzQ=.jpg)
![](http://pic.ikafan.com/imgp/L3Byb3h5L2h0dHAvaW1hZ2UxMDkuMzYwZG9jLmNuL0Rvd25sb2FkSW1nLzIwMjEvMDMvMDIyMS8yMTY3MDgzMzBfMl8yMDIxMDMwMjA5NDUyMzMzMw==.jpg)
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注意:每个数前面的运算符号是这个数的符号.如+46,-125,+54.而325前面虽然没有符号,应看作是+325。
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几个比较接近于某一整数的数相加时,选这个整数为“基准数”。
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例10:计算 78+76+83+82+77+80+79+85
=640
解:
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