1/1×2＋1/2×3＋1/3×4＋1/4×5＋1/5×6
=(1/1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/5-1/6)
中间正负抵消
=1-1/6
=5/6
1/2=1-1/2
1/6=1/2-1/3
……
1/30=1/5-1/6
∴原式=1-1/6=5/6
1/2+1/6+1/12+1/20+1/30
=1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)
=1-1/6
=5/6（六分之五）
1/2 +(1/6 +1/12)+ (1/20 +1/30)两个两个结合然后通风就好了
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