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

　　1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=?

　　1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90

　　=1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+1/7*8+1/8*9+1/9*10

　　=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

　　=1/2-1/10

　　=2/5