10×11/1+11×12/1+12×13/1
=10/1-11/1+11/1-12/1+12/1-13/1
=10/1-13/1
=130/3
2×3/1+3×4/1————99×100/1
=2/1-3/1+3/1-4/1+.+99/1-100/1
=2/1-100/1
=100/49