I’ll draw this as best I can:

 

Look OK? Now let me draw a few more segments in blue…

See what’s going on there? All of the small triangles in the figure are the same! (You can prove this with triangle similarity/congruence rules easily enough—I won’t spend the time doing so here, though.)

We know that the area of the big triangle is (½)(10)(10) = 50. Further, we know that of the 8 small congruent triangles in the figure, 4 of them are inside the square and 4 aren’t. Because those 8 triangles together have an area of 50, the square has an area of half that: 25.

from Tumblr https://ift.tt/2vZoYXb