1+2-3+4-5+6-7+.+98-99+100
=1+(2-3)+(4-5)+(6-7)+.+(98-99)+100
=1-1-1-1-.-1+100
=1+100-1-1-1-.-1 共100项,除开第一个及最后一个外,还有98个,以两个为一组,共98/2=49组
=101-49
=52
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