已知A=3x平方+3y平方-5xy,B=2xy-3y平方+4x平方

∵A=3x^2+3y^2-5xy,B=2xy-3y^2+4x^2
①2A-B
2（3x^2+3y^2-5xy）－2xy-3y^2+4x^2
=6x^2+6y^2-10xy-2xy+3y^2-4x^2=2x^2-12xy+9y^2
②2B-A
2(2xy-3y^2+4x^2)-3x^2+3y^2-5xy
=4xy-6y^2+8x^2-3x^2-3y^2+5xy=5x^2+9xy-9y^2
③A+B-(A-B)=A+B-A+B=2B
2(2xy-3y^2+4x^2)
=8x^2+4xy-6y^2