I know that 35 unique quadrilaterals can be formed from a heptagon by joining the vertices. I can work that out in a diagram. What I want to know is the math behind figuring this out. An equation that will work with any shape (octagon for instance), and/or how this equation is derived. Detailed explanation please.

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^ Combinations formula. Seems legit.

I do understand that, but I want to know how this can be solve with out the use of a calculator.

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