用作差法比较大小
M-N
=5x^2-2x+3-3x^2+3x-x^2-x-2
=x^2+1≥1＞0
所以M＞N
A=-3x³+2x²-1,B=x³-2x²-x+4
2A-（A-B)
=2(-3x^3+2x^2-1)-(-3x^3+2x^2-1-x^3+2x^2+x-4)
=-6x^3+4x^2-2-(-4x^3+4x^2+x-5)
=-6x^3+4x^2-2+4x^3-4x^2-x+5
=-2x^3-x+3
当x=-1时,原式=-2×(-1)+1+3=6
(-1/3a²b)-1/2ab²+(-1/4ba²)-(-2b²a)
=-1/3a^2b-1/2ab^2-1/4a^2b+2ab^2
=(-1/3-1/4)a^2b+(2-1/2)ab^2
=-7/12a^2b+3/2ab^2
3a²-【6a-(4a-3)-2a²】
=3a^2-(6a-4a+3-2a^2)
=3a^2-(-2a^2+2a+3)
=3a^2+2a^2-2a-3
=5a^2-2a-3