联立:m=4-n
n+2=m+2
解得:m=n=2
(1)0.5b-0.4a+0.2(a-b)
=0.5b-0.4a+0.2a-0.2b
=0.3b-0.2a
(2)(3^2x^2y)/5
系数9/5;次数3
(3)系数4;次数5