设y=f(x)=a(x-1)(x-3)且点(0,-3)在图像上,代入解得 a=-1
所以f(x)=-x^2+4x-3
Y=f(x)+ax=-x^2+4x-3-x
=-x^2+3x-3
=-(x-1.5)^2-(3/4)
当x∈[2,+∞]时,Ymin=-∞