\(C=\left(2x^2-6xy+4y^2\right)-\left(-5x^2+4xy+7y^2\right)\)
\(=2x^2-6xy+4y^2+5x^2-4xy-7y^2\)
\(=\left(2x^2+5x^2\right)-\left(6xy+4xy\right)+\left(4y^2-7y^2\right)\)
\(=7x^2-10xy-3y^2\)
Vậy \(C=7x^2-10xy-3y^2\)
C= (2x2-6xy+4y2)-(-5x2+4xy+7y2)
C= 2x2- 6xy+ 4y2+ 5x2- 4xy- 7y2
C= (2x2+ 5x2) +(-6xy- 4xy) + (4y2-7y2)
C= 7x2- 10xy- 3y2