三级训练数学题

显示全部楼层 · 2008-6-13 18:03:45

12分之1加20分之1加30分之1加42分之1加56分之1加72分之1等于多少？

千问 · 2008-6-13 18:03:45

1/12=1/3-1/41/20=1/4-1/51/30=1/5-1/61/42=1/6-1/71/56=1/7-1/81/72=1/8-1/9上面六个式子相加，右边刚好剩下1/3-1/91/12+1/20+1/30+1/42+1/56+1/72＝1/3-1/9＝2/9

千问 · 2008-6-13 18:03:45

12分之1加20分之1加30分之1加42分之1加56分之1加72分之1=1/3*4+1/4*5+....+1/8*9=1/3-1/4+1/4-1/5+...+1/8-1/9=1/3-1/9=2/9

千问 · 2008-6-13 18:03:45

原式=1/3*4+1/4*5+....+1/8*9 =1/3-1/4+1/4-1/5+...+1/8-1/9 =1/3-1/9 =2/9

千问 · 2008-6-13 18:03:45

原式=1/(3*4)+1/(4*5)+^^^^^+1/(8*9)
=1/3-1/4+1/4-1/5+……+1/8-1/9
=1/3-1/9
=2/9

千问 · 2008-6-13 18:03:45

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