1.3xy-4xy-(-2xy)=3xy-4xy+2xy=(3-4+2)xy=xy
2.-ab/3-a^2/4+a^2/3-(-2ab/3)=-ab/3-a^2/4+a^2/3+2ab/3
=(-1+2)ab/3+(-1/4+1/3)a^2
=ab/3+a^2/12
3.(-x+2x^2+5)+(4x^2-3-6x)
=-x+2x^2+5+4x^2-3-6x
=6x^2-7x+2
4.(3a-ab+7)-(-4a^2+2ab+7)
=3a-ab+7-4a^2-2ab-7
=-a^2-3ab+3a
5.5(3a^2b-ab^2)-(ab^2+3a^2b)
=15a^2b-5ab^2-ab^2-3a^2b
=12a^2b-6ab^2
当a=1/2,b=1/3时
12a^2b-6ab^2
=12×(1/2)^2×1/3-6×1/2×(1/3)^2
=12×1/4×1/3-6×1/2×1/9
=1-1/3
=2/3