500-1-2-3-········-76-77-78.怎么算?要巧算.
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500-1-2-3-········-76-77-78.怎么算?要巧算.500-1-2-3-········-76-77-78.怎么算?要巧算.500-1-2-3-········-76-77-78.
500-1-2-3-········-76-77-78.怎么算?要巧算.
500-1-2-3-········-76-77-78.怎么算?要巧算.
500-1-2-3-········-76-77-78.怎么算?要巧算.
因为1 + 2 + 3 +---- +76+77+78
和78+77+76+-----+3 + 2 + 1 两式的和相等,并且以上两式对应上下相加1+78=79,有78个79.所以一个式子的和为(1+78)×78÷2.故
500-1-2-3-········-76-77-78
=500-(1+2+3+----+76+77+78)
=500-(1+78)×78÷2
=500-79×39
=500-3081
=-2581
500-(1+78)*78/2=-3144
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