解:公式:1/n-1/(n+2)=2/n(n+2)
所以:1/n(n+2)=1/2×[1/n-1/(n+2)]
1/(13×15)+1/(15×17)+1/(17×19)+...........+1/(35×37)+1/(37×39)
=1/2(1/13-1/15)+1/2(1/15-1/17)+1/2(1/17-1/19)+...+1/2(1/35-1/37)+1/2(1/37-1/39)
=1/2(1/13-1/15+1/15-1/17+1/17-1/19+...+1/35-1/37+1/37-1/39)
=1/2(1/13-1/39)
=1/2×(2/39)
=1/39
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