range set of f(x,y) values
Limits if L1=L2, lim exists
Common paths axes, x=y, (match exponents)
if f(x,y) is continuous at (a,b) the lim (x,y)>>(a,b) exists
first order partial derivatives f_x, f_y, f_z
second order partial derivatives f_xx, f_xy, f_yx, f_yy
partial derivatives are slopes of the tangent line at a given point to the traces C_1 and C_2