Uitleg vlakke figuren https://www.youtube.com/watch?v=nbSgdPCY9Ks
Vierkanten https://www.youtube.com/watch?v=14ZWgwIhkZw&list=PLqmYEL-9zWjOHe42KExqEuhMJtZPAYco5
Rechthoeken https://www.youtube.com/watch?v=8hYZwe4oVDo&list=PLqmYEL-9zWjOHe42KExqEuhMJtZPAYco5&index=2
Diagonalen https://www.youtube.com/watch?v=4Y3ddgjsAZc&list=PLqmYEL-9zWjOHe42KExqEuhMJtZPAYco5&index=3