Hello, leart369!

Label the inner right triangle like this:

Code:

A
*
| * c
b | *
| *
* - - - - - - - * B
C a

Note that for any triangle, its area is: .

. . (One-half the product of two sides and the sine of the included angle)

The right triangle at the lower-left has sides and .

. . Its area is: .

The triangle at the right has sides and , and included angle

. . Its area is: .

In the above diagram, we see that: .

. . Hence: .

The third triangle has sides and , and included angle

. . Its area is: .

In the above diagram, we see that: .

. . Hence: .

Fascinating! . . . All **four** triangles have the same area.