Question: A 1.85 m tall man is walking toward a 12 m tall street light at night at a rate of 2.2 m/s. How fast is the length of his shadow changing when he is 12 m from the street light? so, using similar triangles, i got that (x+y)/12 = x/1.85. I can rearrange this into x= or y=, but then I don't see how i can find the derivative of it (ex. x = (1.85x+1.85y)/12 -> finding derivative will eliminate x and y). Any help will be greatly appreciated. Thanks!