1/(5×6) + 1/(6×7) + 1/(7×8)
= (1/5 - 1/6) + (1/6 - 1/7) + (1/7 - 1/8)
= 1/5 - 1/8
= (8 - 5) / (5×8)
= 3/40
=5/6+6/7+7/8
=1-1/6+1-1/7+1-1/8
=3-(1/6+1/7+1/8)
=3-73/168
=2又168分之95
=1/5-1/6+1/6-1/7+1/7-1/8=1/5-1/8=3/40
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