第1个回答 2011-04-25
1、(1/1x2)+(1/2x3)+(1/3x4)+(1/4x5)…………+(1/7x8)
=(1-1/2)+((1/2-1/3))+(1/3-1/4)…………+(1/7-1/8)
=1-1/8
=7/8
2、2000x1999-1999x1998+1998x1997-1997x1996+……+2x1
=(2000×1999-1999×1998)+(1998×1997-1997×1996)……+2×1
=1999×2+1997×2+1995×2+……+1×2
=(1999+1997+1995+……+1)×2
=(1999+1)×[(1999-1)÷2+1]÷2×2
=2000×1000÷2×2
=2000000
3、0.1(1循环)+0.21(21循环)+0.31(31循环)+……+0.81(81循环)+0.91(91循环)
=11/99+21/99+31/99+……+81/99+91/99
=459/99
=51/11
第2个回答 2011-04-25
(1/1x2)+(1/2x3)+(1/3x4)+(1/4x5)…………+(1/7x8)=(1-1/2)+(1/2-1/3)+(1/3-1/4) +(1/4-1/5)+……+(1/7-1/8))=1-1/8=7/8
2000x1999-1999x1998+1998x1997-1997x1996+……+2x1
=2000×1999-1999×1998)+(1998×1997-1997×1996)……+2×1
=1999×2+1997×2+1995×2+……+1×2
=(1999+1997+1995+……+1)×2
=(1999+1)×[(1999-1)÷2+1]÷2×2
=2000×1000÷2×2
=2000000
0.1(1循环)+0.21(21循环)+0.31(31循环)+……+0.81(81循环)+0.91(91循环)
=11/99+21/99+31/99+……+81/99+91/99
=459/99
=51/11
第3个回答 2011-04-28
貌似已经有人回答了,我挣分吧~~~~~~~~~~~~