Question

What’s the proof for this

Answer 1

Well... Basically, you should prove this by SSS property(side-side-side). It's fair to say that the length of a side is equal to itself so The line that cuts through the rectangle is a side for both rectangles. Thus because of the given, all the sides of the triangles are equal to one another. This is a very important trick for geometry(I remember using it a lot).
Hope this helps!