1/2+1/6+1/12+1/20+1/30+1/42+1/50+1/72+1/90=1/1*2+1/2*3+1/3*4+...+1/9*10
=1-1/2+1/2-1/3+...+1/9-1/10
=1-1/10
=9/10
1又1/3-7/12+9/20-11/30+13/42-15/56
=1+1/3-(1/3+1/4)+(1/4+1/5)+...-(1/7+1/8)
=1+1/3-1/3-1/4+1/4+1/5+...-1/7-1/8
=1-1/8
=7/8