We know what dy dt is right at this moment. Can we come up with a relationship between x and y and maybe take the derivative with respect to t so we can figure out what dx dt is at a given moment in time? Well, both of these triangles, and when I say both of these triangles, let me be clear what I'm talking about. This triangle right over here, the smaller triangle in green, is a similar triangle to the larger triangle. It's a similar triangle to this larger triangle that I am tracing in blue. It's similar to this larger one. How do I know that?