y=x^3+3axy在x处的导数y'=3x^2+3a （即使切线斜率）所以过(b,b^3+3ab)的切线为y-b^3-3ab=(3b^2+3a)(x-b)y=(3b^2+3a)x-2b^3因为切线y=3x+1所以-2b^3=1 b=-(1/2)^(1/3)3b^2+3a=3(1/2)^(2/3)+a=1a=1-(1/2)^(2/3)...