A Dynamic View of Lagrange Multipliers
You see a contour map of a function $z = f(x,y)$; The light grey curves are curves of constant $z$ value, spaced 10 apart. There is also a path (the red ellipse) $g(x,y) = c$ for some constant value $c$. The dark blue curve is a contour of $f$ whose $z$ value you can vary by using the slider. There is a point $A$ on the path showing unit vectors in the directions of the gradient of the functions $f$ and $g$.