如题所述
11×2/13，这是不是11×13是分母，2是分子？
如果是则
2/(11×13)+2/(13×15)+2/(15×17)+2/(17×19)+1/19
=(13-11)/(11×13)+(15-13)/(13×15)+(17-15)/(15×17)+(19-17)/(17×19)+1/19
=(1/11-1/13)+(1/13-1/15)+(1/15-1/17)+(1/17-1/19)+1/19
中间正负抵消
=1/11
如果是则
2/(11×13)+2/(13×15)+2/(15×17)+2/(17×19)+1/19
=(13-11)/(11×13)+(15-13)/(13×15)+(17-15)/(15×17)+(19-17)/(17×19)+1/19
=(1/11-1/13)+(1/13-1/15)+(1/15-1/17)+(1/17-1/19)+1/19
中间正负抵消
=1/11
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