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Count Number of Triangles

From a recent homework in Level I session:

Step 1: Count the number of triangles with vertices pointing up:

    count with size = 1: 1 + 2 + 3 ... + n (vertices in each layer) = $$\sum_{k=1}^N k(N-k+1)$$

Related Problems

• Applicable to WK3 #6, 7 — Quadratic Factorization
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