Three similar right triangles

There are three right triangles in this diagram, and they are all similar. Can you identify them?[br][br]To see that the small and medium triangles are similar, click the check box at top left, then slide the sliders to the right, in order from top to bottom.[br][br]Repeat with the right hand check box for the medium and large triangles.
Can you prove this?[br]Hint: click the "Show angles" check box.

Information: Three similar right triangles