Hi Folks,

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

$\displaystyle f_1=f(0,0)=c_1$

$\displaystyle f_2=f(a,0)=c_1+c_2a$

$\displaystyle f_3=f(a,b)=c_1+c_2a+c_3b+c_4ab$

$\displaystyle f_4=f(0,b)=c_1+c_3b$

How are these equations determined? See attached pic

Thanks