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!
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An equilateral triangle is NOT concave. The sides only push out, and they never cave in. An equilateral triangle is convex. I hope that helped all who needed.
concave
A trapezoid is convex.
convex
A non convex is a concave and a convex is differently shaped