Isn't it simple counting?

Let \(A_{1}A_{2}A_{3}A_{4}.....A_{21}\) be a 21-sided regular polygon inscribed in a circle with centre \(O\). How many triangles \(A_{i}A_{j}A_{k}\), \(0<i<j<k<22\), contain the point \(O\) in their interior.

NOTE: This is not an original problem

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