(1)4x-5x=(4-5)x=-x
(2)4a-5a+8a=(4-5+8)a=7a
（3）-2x+3y+7x-二分之一y=-2x+7x+3y-y/2=5x+5y/2
（4）5mm-2m+4mm+6m-2=5m²+4m²-2m+6m-2=9m²+4m-2