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This game featured in the NRICH Primary and Secondary webinar in April 2023.
This printable worksheet may be useful: Rhombus It
Start with a demonstration playing against a student rather than against the computer. Students may be surprised when the winning rhombus isn't aligned with the grid. This leads to discussions about what makes a rhombus a rhombus.
After a demonstration of the game, students could be left to play for a while in pairs, either on the computer or on a paper grid. Give them the option of reducing the size of the board if they seem overwhelmed!
Bring the class together for a discussion of their thoughts on the game. Did anyone consistently win or lose? Can anyone think of any good strategies which might help them win?
Once ideas have been shared the group can return to playing in pairs, or they can play a game together against the computer, trying, as a class to decide on the best move at each stage. Ask each student to explain the reasoning behind the moves they choose.
One aspect of developing a winning strategy that could be considered is the number of distinctly different starting points ($6$ on a $5 \times 5$ board) and the number of different rhombuses that can be drawn that include each of those points. That is, "Is there a good place to start and why?". This is a great investigation, with the capacity to expand by changing the sizes of the starting
grid, and which leads back into the game itself.
Working on the properties of a rhombus offers an opportunity to look at gradients to establish whether a shape is a rhombus.
With classes who never arrive together or on time, this and other interactive games can be used on the Interactive White Board to engage the early arrivers and set up a relaxed mathematical atmosphere.