2/97＋4/97＋6/97＋8/97＋……＋98/97＋100/97
= 2/97×(1+2+3+...+49+50)
= 2/97×(1+50)×25
= 2550/97
把 1/2＋1/3＋1/4 看成一个整体,即设 1/2＋1/3＋1/4 = a,则
(1＋1/2＋1/3＋1/4）×（1/2＋1/3＋1/4＋1/5）－（1+1/2＋1/3+1/4+1/5）×（1/2+1/3+1/4）
= (a+1)(a+1/5) - (a+1+1/5)*a
= (a+1)*a+(a+1)*1/5 - (a+1)*a - 1/5*a
= (a+1)*1/5 - 1/5*a
= 1/5*(a+1-a)
= 1/5
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