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The diagram shows nine points in a square array.

What is the smallest number of points that need to be removed in order that no three of the remaining points are in a straight line?


 

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.

 

 

This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.