若关于x,y的单项式cx^(2a+2)y²与0.4xy^(3b+4)的和为零,则两个单项式不仅是同类项,而且两个单项式的系数互为相反数,所以有：
2a+2=1
3b+4=2
c+0.4=0
解得 a=-1/2,b=-2/3,c=-2/5
原式=1/2a²b-[2/3a²b-(3abc-a²c)-4a²c]-3abc
=1/2a²b-(2/3a²b-3abc+a²c-4a²c)-3abc
=1/2a²b-2/3a²b+3abc-a²c+4a²c-3abc
=-1/6a²b+3a²c
=(-1/6)×(-1/2)²×(-2/3)+3×(-1/2)²×(-2/5)
=(-1/6)×1/4×(-2/3)+3×1/4×(-2/5)
=1/36-3/10
=5/180-54/180
=-49/180