（1）1/1*2+1/2*3+1/3*4+1/4*5
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5
=1-1/5
=4/5
（2）1/2+1/6+1/12+1/20+1/30+1/42
=1/1*2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7
（3）1+1/3+1/6+1/12+1/10+1/15+1/21
=2x(1/2+1/6+1/12+1/20+1/30+1/42) （括号内（2）已算）
=2x6/7
=12/7