ax = f, cx = g とおくと


|a, b, c, d|
|b, f, d, g|
|c, d, ayy, byy|
|d, g, byy, fyy|

= {(af-bb)yy + (cg-dd)}^2 - (ag+cf-2bd)^2 yy

= {(af-bb)yy +(ag+cf-2bd)y +(cg-dd)}{(af-bb)y^2 -(ag+cf-2bd)y +(cg-dd)},