第1个回答 2009-02-19
1、1/2+1/3-1/3-1/4+1/4+1/5+……+1/20+1/21-1/21-1/22
1/2-1/22=5/12
2、1+1/3-1/3-1/4+/4+1/5+……-1/7-1/8
=1-1/8=7/8
4、2890÷(5/6+7/8+7/10)+( 5/6+7/8+7/10)÷(5/6+7/8+7/10)
=1200+1=1201
第2个回答 2009-02-19
1.1/2+1/3-1/3-1/4+1/4+...-1/21+1/22=1/2+1/22=6/11
2.同上 1-1/8=7/8
第3个回答 2009-02-19
1: 5/6-7/12+9/20-11/30+13/42-15/56......+14/420-43/462
=(2+3)/(2*3)-(3+4)/(3*4)+(4+5)/(4*5)-…………
=(1/2+1/3)-(1/3+1/4)+(1/4+1/5)- ………… +(1/20+1/21)-(1/21+1/22)
=1/2-1/22
=5/11
2:1又1/3-7/12+9/20-11/30+13/42-15/56
=4/3-7/12+9/20-11/30+13/42-15/56
=1/2+5/6-7/12+9/20-11/30+13/42-15/56
=1/2+(2+3)/(2*3)-(3+4)/(3*4)+(4+5)/(4*5)-…………
=1/2+(1/2+1/3)-(1/3+1/4)+(1/4+1/5)- …………+(1/6+1/7)-(1/7+1/8)
=1/2+1/2-1/8
=7/8
3:1/1+2 + 1/1+2+3 + 1/1+2+3+4 +......+1/1+2+......+99
1+2+3+……+n=n*(n+1)/2
1/n*(n+1)=1/n-1/(n+1)
1/1+2 + 1/1+2+3 + 1/1+2+3+4 +......+1/1+2+......+99
=2(1/2*3 + 1/3*4 + ………… + 1/99*100)
=2(1/2-1/3 + 1/3-1/4 + 1/4-1/5 + ………… + 1/98-1/99 +1/99-1/100)
=2(1/2-1/100)
=49/50
4:(2890+5/6+7/8+7/10)÷(5/6+7/8+7/10)
5/6+7/8+7/10=289/120
(2890+5/6+7/8+7/10)÷(5/6+7/8+7/10)
=2890÷(5/6+7/8+7/10)+1
=2890*120/289+1
=1201