1/1x2x3+1/2x3x4+1/3x4x5+......+1/8x9x10 =1/2 [1/1×2-1/2×3＋1/2×3-1/3×4＋1/3×4-1/4×5＋。。。＋1/8×9-1/9×10]=1/2[1/2-1/90]=1/2×44/90=11/45
