# Thread: The equation of a hyperbolic paraboloid to derive the corner points of rectangle

1. ## The equation of a hyperbolic paraboloid to derive the corner points of rectangle

Hi Folks,

I have come across some text where $f(x,y)=c_1+c_2x+c_3y+c_4xy$ is used to define the corner points

$f_1=f(0,0)=c_1$
$f_2=f(a,0)=c_1+c_2a$
$f_3=f(a,b)=c_1+c_2a+c_3b+c_4ab$
$f_4=f(0,b)=c_1+c_3b$

How are these equations determined? See attached pic

Thanks