p = -9/4
x^4+6x^3+9x^2-3px^2-9px+2p^2
可因式分解:
= (x^4+6x^3+9x^2) - 3p(x^2+3x) +2p^2
= (x^2+3x)^2 - 3p(x^2+3x) +2p^2
= (x^2+3x -p) (x^2+3x -2p)
所以
x^2+3x -p = 0 或 x^2+3x -2p =0
若 1 有重根,x1+x2 = -3 ,x1=x2= -3/2,-p= x1*x2 = 9/4,p = -9/4
若 2 有重根,x1+x2 = -3 ,x1=x2= -3/2,-2p = x1*x2 = 9/4,p = -9/8 （经检验,导致1式有解,舍去)