Hi All, This question is about vector calculus, gradient, directional derivative and normal line. If the gradient is the direction of the steepest ascent: >> gradient(x, y) = [ derivative_f_x(x, y), derivative_f_y(x, y) ] Then it really confuse me as when calculating the normal line perpendicular to the tangent plane, the formula would be: >> normal line = (derivative_f_x(x, y), derivative_f_y(x, y), z), But both derivative_f_x(x,y) & derivative_f_y(x,y) are gradient (the slope of the tangent plane). I don't think the steepest ascent/descent is the slope of the normal line perpendicular to the tangent plane! For example Find a vector function for the line normal to x^2 + 2y^2 + 4z^2 = 26 at (2, -3, -1). Answer: (2 + 4t, -3 -12t, -1 - 8t). Anyone care to give it a shot and show me the step?? Any information would be much appreciated. Thanks.