分式与的最简公分母是____________.
计算:
(1)+=____________
(2)()2=_____________
(3)+=_______________
(4)-=____________
(5)1÷(+)=_________________
约分:(1)=_________________(2)=________________
当x=1,y=-4时,分式的值是____________.
当x=__________时,分式有意义.
因式分【解析】
(1)9-4a2=________________
(2)-15x3y2+10x2y4-20x4y3=__________________
(3)a2-5ab+6b2=__________________
(4)-m2+4mn-4n2=________________________