Is a triangle convex or concave?

Answer 1

Three vertices polygons (aka triangle) can not be concave or convex because it doesn't have enough verticies to susustain an internal angle is less than or equal to 180 degrees.

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Actually, a triangle IS a convex polygon. Convexity requires angle to be less than 180 degrees and since the three angles of a triangle sum to 180 degrees, each one of them MUST be less than 180 degrees!