Brilliant.org has a nice area problem for a square of edge 4 and a pair of similar triangles formed by an edge of length 1 crossing to the opposite side. The original question was what portion of the square is the two triangles. That generalizes pretty quickly, and Danny Brown (@danieltybrown) thought to look at the ratios of the areas (pix below), which is also interesting, so... off to GeoGebra.
Original problem: brilliant.org