Geometry card tricks

There are many mathematical theories that can be explored within the world of Dobble. In this page, there are some of those geometries and theories you can learn about, while playing around with the interactive applets and games.

Pappus's Theorem

Pappus's theorem states that given one set of collinear points A,B,C, and another set of collinear points a, b, c, then the intersection points X, Y, Z of line pairs Ab and aB, Ac and aC, Bc and bC are collinear, lying on the Pappus line.

Play around with Pappus

Use the following applet produced in GeoGebra to play around the with the points on the lines to see Pappus's theorem in action.

Dobble card trick with Pappus Theorem

Let's try Pappus's Theorem with Level 5 Dobble!

Drag the cards from the cards containers into the black circle(points) on the lines.

You can click on a symbol(letter) to highlight the cards that contains the chosen symbol and click on two symbols to highlight the card that contains both of those symbols. Similarly, you can click on a card to highlight the symbols that the card conatins and click on two cards to highlight the symbol that the both of those cards contains.

If the three cards on a line has a common symbol, the symbol will be shown next to the line to verify you have chosen the right cards to form the line, indicating that you are in the right path.

If you think you have chosen a wrong card, you can replace it with another card on top of the "wrong card" or start again using the "Start Again" button.

Happy Pappus-Dobble playing!