1题：
1-1/6+1/42+1/56+1/72
=1-1/6+(1/42+1/56+1/72)
=1-1/6+[(1/6-1/7)+(1/7-1/8)+(1/8-1/9)]
=1-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1-1/9
=8/9
2题：
2/(2×4)+2/(4×6)+2/(6×8)+2/(8×10)
=(1/2-1/4)+(1/4-1/6)+(1/6-1/8)+(1/8-1/10)
=1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10
=1/2-1/10
=5/10-1/10
=2/5
3题：
4/(5×9)+4/(9×13)+4/(13×17)+4/(17×21)
=(1/5-1/9)+(1/9-1/13)+(1/13-1/17)+(1/17-1/21)
=1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21
=1/5-1/21
=21/105-5/105
=16/105