1、解:(1)原式=6/2x-1/2x=5/2x;
(2)原式=2y/x-1-y/x-1=-3y/x-1.
2、解:(1)c/a+c/b=ac+bc/ab.
(2)1/x+1-1/x-1
=x-1-x-1/(x+1)(x-1)
=-2/x²-1.
(3)a/b-b/a-a²+b²/ab
=a²/ab-b²/ab-a²+b²/ab
=a²-b²-(a²+b²)/ab=-2b²/ab=-2b/a.
(4)1/3x+1/2y-x+2y/6xy
=2y/6xy+3x/6xy-x+2y/6xy
=2y+3x-x-2y/6xy
=1/3xy.
3、解:(1)原式=4/ 6x²-5x/6x²=4-5x/6x².
(2)原式=2a/a² -4b² -a+2h/a² -4b²=2a-a-2h/a² -4b²
=a-2b/a²-4b²=1/a+2b.
4、解:七年级(2)班清除全部杂草用m/an小时;
两班合作时每小时清除杂草an+ bk,清除全部杂草用m/an+bk小时.
因为m/an-m/an+bk=man+mbk-man/an(an+bk)=mbk/an(an+bk),
所以提前mbk/an(an+bk)小时