**Puzzle: **In the following figure, what is the area of the green
square?

**Solution: **First, observe that the largest right triangle (colored
blue in the following diagram) measures r by 1 by sqrt(r^{2}+1), in
order of their side lengths.

Next, observe that the slightly smaller right triangle (colored blue near
the bottom of the next illustration) is similar to the first, and so it
measures r/sqrt(r^{2}+1) by 1/sqrt(r^{2}+1) by 1, in order of
increasing side lengths.

The area of the smaller blue triangle is half the product of the legs,
(1/2) r/(r^{2}+1). Four such triangles are subtracted from 1 to
get the area of the small square: 1-2r/(r^{2}+1).

### Internet references

Mathworld: Square,
which contains this puzzle

### Related pages in this website:

Summary of geometrical theorems

The webmaster and author of this Math Help site is
Graeme McRae.