2a²b-3a²b+2分之1a²b
=(2-3+1/2)a^2b
=-1/2a^2b
a的立方-a²b+ab²+a²b-ab²+b的立方
=a^3+(-1+1)a^2b+(1-1)ab^2+b^3
=a^3+b^3.