If then, the tangent plane to at is
i have a suface
find the formula of a plane which is tangent to it in (x0,y0,z0)
i need to find the vector whic is vertical to this surface
the formula of it is
but i dont know what is the formula for f(x,y) ,
i have a formula in other form
how to write it in the f(x,y) form
?