236. A cube with edge $a$ is standing on a plane. The light source is located at a distance $b(b>a)$ from the plane. Find the minimum value of the area of the shadow cast by the cube on the plane.
236. Note that the shadow cast by only the upper face of the cube (assuming all other faces are transparent) is a square with side $\frac{a b}{b-a}$. From this, it follows that the area of the shadow cast by the cube will be the smallest when the light source is positioned above the upper face (only the upper face of the cube is illuminated); it will be equal to $\left(\frac{a b}{b-a}\right)^{2}$, taking into account the area of the lower face of the cube.