Given this function: f(x,y) = a) How large is the directional derivative at the point (-1,2) in the direction of the origin. b) In which direction does f increase most in the point (-1.2)? How big is the directional derivative in this direction? c) Find the stationary / critical points of the function. d) Classify the stationary / critical points. f) Find a formula for the tangent plane of the function in the point (1,1,2). g) Use the formula for the tangent plane to approximate the value of the function in the point (x, y) = (0.9, 1.1). How big is the difference between approximate and exact value.