1：1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90
=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=7/30
2：1/5×7 + 1/7×9 + 1/9×11 + 1/ 11×13 + 1/ 13×15
=1/2×(1/5-1/7)+1/2×(1/7-1/9)+1/2×(1/9-1/11)+1/2×(1/11-1/13)+1/2×(1/13-1/15)
=1/2×(1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+1/13-1/15)
=1/2×(1/5-1/15)
=1/2×2/15
=1/15
3： 6/1×7 － 6/7×13 － 6/13×19－ ．．．－6/97×103
=(1-1/7)-(1/7-1/13)-(1/13-1/19)-.-(1/97-1/103)
=1-1/7-1/7-1/13+1/13-1/19-.-1/97+1/103
=1-1/7-1/7+1/103
=5/7+1/103
=522/721