=1/3[1/(1*2*3)-1/(2*3*4)+1/(2*3*4)-1/(3*3*5)+..-1/(7*8*9)+1/(7*8*9)-1/(8*9*10)]
=1/3(120/720-1/720)
=119/2160
1/3(1/1*2*3-1/2*3*4)+1/3(1/2*3*4-1/3*4*5)+.......+1/3(7*8*9-1/8*9*10)
=1/3(1/1*2*3-1/8*9*10)
=1/3(1/6-1/720)
=119/2160
希望能理解 愿学习进步
400*39/70
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