Catriona Shearer posted this problem to Twitter and it got a lot of discussion, so I thought I’d post my solution here in more detail than Twitter permits.
This design is made of three 2×1 rectangles. What fraction of it is shaded?
Label vertices and draw in a couple line segments:
Considering triangles CFJ and CKJ, angles CFJ and CKJ are right, line segment CJ is common, and CF = CK = 2, so the two triangles are congruent and FJ = JK = 1. But then angles CGE and JGK are equal, angles CEG and GKJ are right, and CE = JK, so those triangles are congruent and CG = GJ.
So JK = 1, and if KG = x, GJ = CG = 2-x, so 1²+x² = (2-x)². The solution is x = 3/4. The area of each shaded triangle is 3/8. The area of the whole pattern is three rectangles minus the shaded area: 6-3/4 = 21/5. The shaded area is 1/7 of the area of the whole pattern.
Notice the shaded triangles are 3:4:5.
Can the shaded area be dissected into pieces, seven of each of which will fill the pattern? Yes.
Here are the relative sizes of the pieces, in case you’re interested:
- Blue: 9:12:15
- Green: 12:16:20
- Red: 9:25:30:38
- Magenta: 10:16:38:40
- Shaded triangles (blue+red and green+magenta): 30:40:50