由 A=3x^2-5xy,B=3xy+x^2,A+B+C=0
3x^2-5xy+3xy+x^2+C=0
4x^2-2xy+C=0
C=2xy-4x^2
2A-5B+3C = 6x^2-10xy-15xy-5x^2+6xy-12x^2=-11x^2-19xy=(11x-19y)x
由 A=3x^2-5xy,B=3xy+x^2,A+B+C=0
3x^2-5xy+3xy+x^2+C=0
4x^2-2xy+C=0
C=2xy-4x^2
2A-5B+3C = 6x^2-10xy-15xy-5x^2+6xy-12x^2=-11x^2-19xy=(11x-19y)x