用待定系数法,先分解x^2-2xy-3y^2=(x-3y)(x+y)
那么原式=（x-3y+A)(x+y+B)=x^2-2xy-3y^2+Ax+Ay+Bx-3By+AB
A+B=3
A-3B=-5
A=1
B=2
原式=（x-3y+1)(x+y+2)
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