1/C(3,n)=2×3/[n(n-1)(n-2)]
=3[1/(n-1)(n-2)-1/n(n-1)]
所以1/C(3,100)+1/(C3,99)+…+1/C(3,3)
=3[1/99×98-1/100×99+1/98×97-1/99×98+……+1/2×1-1/3×2]
=3[-1/100×99+1/2×1]
=3[1/2-1/9900]
=4949/3300