Find all points at which the direction of fastest change of the function f(x,y) = x² + y²-4x-8y is i+j. (Enter your answer as an equation. Find equations of the tangent plane and the normal line to the given surface of the specified point.x + y + z = 2e, (0,0,2) (a) the tangent plane (b) the normal line (x(t), x(t),z(t)) = ( )