All angles < 180 degrees are concave.
An acute angle is < 90 degrees.
So, all acute angles are concave.
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concave
Angle of contact is the angle between a tangent to the liquid surface at the interface and the solid surface inside the liquid. For concave surfaces, it is acute . For convex surface , it is obtuse.
Any polygon that has an angle that is > 180º is a concave polygon. A convex polygon does not. e.g. All regular polygons are convex.
A simple polygon that is not convex is called concave, non-convex or reentrant. A concave polygon will always have an interior angle with a measure that is greater than 180 degrees.
A concave polygon has at least one interior angle greater than 180 degrees. A convex polygon has none of those.