1/30+1/42+1/56+1/72
=1/（5x6)+1/(6x7)+1/(7x8)+1/(8x9)
=(6-5)/5x6+(7-6)/6x7+(8-7)/7x8+(9-8)/8x9
=(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9) (这里注意是咋来的）
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9（中间几组直接抵消了）
=4/45
1/30+1/42+1/56+1/72
=1/（5x6)+1/(6x7)+1/(7x8)+1/(8x9)
=(6-5)/5x6+(7-6)/6x7+(8-7)/7x8+(9-8)/8x9
=(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9) (这里注意是咋来的）
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9（中间几组直接抵消了）
=4/45
1/30+1/42+1/56+1/72
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=4/45
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