1/(1*2*3)+1/(2*3*4)+1/(3*4*5)+1/(4*5*6)+.+1/(24*25*26)
=1/2*(1/1*2-1/2*3)+1/2*(1/2*3-1/3*4)+……+1/2*(1/24*25-1/25*26)
=1/2*(1/1*2-1/2*3+1/2*3-1/3*4+……+1/24*25-1/25*26)
=1/2*(1/1*2-1/25*26)
=1/2*(1/2-1/650)
=81/325
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