Now, given this setup, can we come up with a relationship between x and y? And then using that relationship, what we're really trying to come up with is what is the rate at which x is changing with respect to time? We know what y is right at this moment. 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.