Consider the hyperbolic paraboloid 2 = I' .2 -y2 . A drone is programmed to fly along the surface of the hyperbolic paraboloid. The flight instruments measure the velocity in the r, y and & directions as dy dt = 3 dt =1 d.z =1 . The GPS measures x = 20, but the GPS does not measure y (to keep costs down). What is the y co-ordinate? The chain rule can tell us! It's Number