(1/1+2)+(1/1+2+3)+(1/1+2+3+4)+.+(1/1+2+3+4+.+49)+(1/1+2+3+4+.+49+50)
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(1/1+2)+(1/1+2+3)+(1/1+2+3+4)+.+(1/1+2+3+4+.+49)+(1/1+2+3+4+.+49+50)
(1/1+2)+(1/1+2+3)+(1/1+2+3+4)+.+(1/1+2+3+4+.+49)+(1/1+2+3+4+.+49+50)
(1/1+2)+(1/1+2+3)+(1/1+2+3+4)+.+(1/1+2+3+4+.+49)+(1/1+2+3+4+.+49+50)
第n项=2/(n+1)(n+2)=2[1/(n+1)-1/(n+2)]
原式=2(1/2-1/3+1/3-1/4+……+1/50-1/51)
=2(1/2-1/51)
=49/51
原式=2×[(1/2-1/3)+(1/3-1/4)+...+(1/50-1/51)]=2×(1/2-1/51)=49/51
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