It is a quadrilateral but I do not know which one it is:
Best Answer
It is a type of kite (Also known as a deltoid). On the left hand side of the image below is a convex kite, and on the right hand side is a concave kite.
Here, we will show that $l, m, n$ are distinct. Suppose not, WLOG say $l=m$, then we get a contradiction since $la = mb \Rightarrow a = b$. Hence, $l \neq m$.
Best Answer
It is a type of kite (Also known as a deltoid). On the left hand side of the image below is a convex kite, and on the right hand side is a concave kite.
Image by Wikipedia:
A concave kite is sometimes referred as a dart or arrowhead.