5/6-7/12+9/20-11/30+13/42-15/56+……+41/420-43/462
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5/6-7/12+9/20-11/30+13/42-15/56+、、、、、、+41/420-43/462
=(1/2+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)+……+(1/20+1/21)-(1/21+1/22)
=1/2+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7+……+1/20+1/21-1/21-1/22
=1/2-1/22
=10/22
=5/11
=(1/2+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)+……+(1/20+1/21)-(1/21+1/22)
=1/2+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7+……+1/20+1/21-1/21-1/22
=1/2-1/22
=10/22
=5/11
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