1/2x3+1/2x3x4+1/3x4x5+.........+1/98x99x100=1/2×(1/1×2-1/2x3+1/2x3-1/3×4+1/3x4-1/4×5+.........+1/98x99-1/99×100)=1/2×(1/1×2-1/99×100)=4949/19800
