Concave shapes are those like the inside of a bowl.
Hollow or depressed shapes - the 'cave' bit should give it to you!
The surface is concave if the liquid is attracted to the container walls and convex if it is not
All 2-dimensional shapes. They may be closed or open, concave or convex, made of straight lines or curved ones, the lines may or may not cross each other.
It depends entirely on their relative shapes and sizes. They could make a triangle, a parallelogram, a square, or a pentagon or even a concave heptagon.
Assuming they don't overlap, and depending on their exact dimensions, these are some of the possible shapes: A triangle (big) A square A rectangle A parallelogram A rhombus A trapezium A pentagon A hexagon A heptagon and if a vertex of the triangle rests on the side of the trapezium, an octagon. a concave pentagon, a trapezium, a concave seven or eight sided figure (neither of the last two is a polygon but two polygons touching one another)
Concave, because you can connect the corners from the outside.