第一题
=4a^2b-(3ab^2-6a^2b+2)
=4a^2b-3ab^2+6a^2b-2
=10a^2b-3ab^2-2
带入ab,得=5
第二题
=1/2x-2x+2/3y^2-3/2x+1/3y^2
=-3x+y^2
带入xy,得=7