3x^2-10xy+3y^2+9x+5y-12=0 (3x-y)(x-3y)-(3x-9y)+12x-4y-12=0 (3x-y)(x-3y)-3(x-3y)+12x-4y-12=0 (x-3y)(3x-y-3)+4(3x-y-3)=0 (x-3y+4)(3x-y-3)=0 所以x-3y+4=0或3x-y-3=0 所以是两条直线