A decagon has 10 sides.
It it is regular you can build 10 isosceles triangles from the center of the decagon to the 10 sides.
Each triangle has a common vertex where the angle of each triangle is 360° / 10 = 36°.
So each time that you rotate the decagon a multiple of 36° around the center you get an image that coincides with the original decagon.
If the letters are given clockwise:
- when you rotate 36° counter clockwise, the point A' (the image of A) will coincide with the point J.
- when you rotate 72° (2 times 36°) counter clockwise, the point A' will land on I.
- when you rotate 108° (3 times 36°) counter clockwise, the point A' will land on H.
- when you rotate 144° (4 times 36°) counter clockwise, the point A' will land on G.
- when you rotate 180° (5 times 36°) counter clockwise, the point A' will land on I.
- when you rotate 216° (6 times 36°) counter clockwise, the point A' will land on E.
- whn you rotate 252° (7 times 36°) counterclockwise, the point A' will coincide with D.
Add other 36° each time and A' will coincide successively with C, B and the same A.
