2/(11x13)+2/(13x15)+2/(15x17)+2/(17x19)+2/(19x21)
=(1/11-1/13)+(1/13-1/15)+(1/15-1/17)+(1/17-1/19)+(1/19-1/21)
=1/11-1/21
=10/(11x21)
=10/231
1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19+1/19-1/21
=1/11-1/21
=10/231
等会
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