1/6=1/2-1/3 1/12=1/3-1/4 1/30=1/5-1/6 1/42=1/6-1/7 1/56=1/7-1/8 1/72=1/8-1/9 1/90=1/9-1/10
所以 1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/2+1/2-1/3+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
=1/2+1/2-1/10
=1-1/10
=9/10
1/6=1/2-1/3 1/12=1/3-1/4 1/30=1/5-1/6 1/42=1/6-1/7 1/56=1/7-1/8 1/72=1/8-1/9 1/90=1/9-1/10
所以 1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/2+1/2-1/3+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
=1/2+1/2-1/10
=1-1/10
=9/10