x^2-5x+3
=x^2-2*(5/2)x+(5/2)^2+3-25/4
=(x-5/2)^2-13/4
=(x-5/2)^2-[(13/4)^(1/2)]^2
=[x-5/2+(13/4)^(1/2)][x-5/2-(13/4)^(1/2)]