：x²－m﹙3x－2m＋n﹚－n²＝0
x^2-3mx+2m^2-mn-n^2)=0
判别=(-3m)^2-4*(2m^2-mn-n^2)
=m^2+4mn+4n^2
=(m+2n)^2>=0
x1=(3m+m+2n)/2=2m+n
x2=(3m-m-2n)/2=m -n