1/1*4+1/4*7+1/7*10+.+1/91*94

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1/1*4+1/4*7+1/7*10+.+1/91*941/1*4+1/4*7+1/7*10+.+1/91*941/1*4+1/4*7+1/7*10+.+1/91*941/1*4+1/4*7+1/7*

1/1*4+1/4*7+1/7*10+.+1/91*94
1/1*4+1/4*7+1/7*10+.+1/91*94

1/1*4+1/4*7+1/7*10+.+1/91*94
1/1*4+1/4*7+1/7*10+.+1/91*94
=1/3 * (1 - 1/4) + 1/3 * (1/4 - 1/7) +...+ 1/3 * (1/91 - 1/94)
=1/3 * (1 - 1/4 + 1/4 - 1/7+...+1/91 - 1/94)
=1/3 * (1 - 1/94)
=31/94

1/1*4 = 1/3 * (1 - 1/4)
1/4*7 = 1/3 * (1/4 - 1/7)
由此规律,
1/1*4+1/4*7+1/7*10+......+1/91*94
=1/3 * (1 - 1/4) + 1/3 * (1/4 - 1/7) +...+ 1/3 * (1/91 - 1/94)
=1/3 * (1 - 1/4 + 1/4 - 1/7+...+1/91 - 1/94)
=1/3 * (1 - 1/94)
=31/94

原式=
(1-1/4+1/4-1/7+...1/91-1/94)/ 3
=(1-1/94)/3
=(93/94)/3 =31/94

31/94