Points A, B and X form a rougly equilateral triangle.
I need to pull a cart from A to B, and also need to visit X, but I don't need to have the cart with me there. Walking with the cart costs me double the energy of walking without.
To spend as little energy as possible, I think I'd need to pull the cart to somewhere near the middle of the triangle (I will call this point R), leave it there, walk to X and back to the cart, and pull the cart on to B.
How can one calculate where exactly R is?