1/2X3X4X5+1/3X4X5X6+1/4X5X6X7......1/97X98X99X100=?
=1/3[1/2*3*4-1/3*4*5]+1/3[1/3*4*5-1/4*5*6]+1/3[1/4*5*6-1/5*6*7]+...+1/3[1/97*98*99-1/98*99*100]
=1/3[1/2*3*4-1/98*99*100]
=1/3*40424/98*99*100
=5053/3*49*99*25
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