1.用+7 +3 -3 +7这四个数组成24点游戏.
7x[3-(-3)÷7]=24
|1/2009-1/2008|+|1/2008-1/2007|-|1/2009-1/2007|
=1/2008-1/2009+1/2007-1/2008-1/2007+1/2009
=0
1又1/3-7/12+9/20-11/30+13/42-15/56
=(1+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)
=1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8
=1-1/8
=7/8
4. c