Determine and name the type of curves arising as level sets of the graph of the function : R R, (x, y) y-x, and as section with the yz-plane and the xz-plane. Write the equation of the tangent plane at the point of coordinates (1,2,1). The level sets have equation hence they are The section with the yz-plane has equation hence it is The section with the xz-plane has equation hence it is The equation of the tangent plane is