Which famous entrepreneur is credited with inventing the weekend?

answer:

an angle is the intersection of two noncollinear rays at a common endpoint. the rays are called sides and the common endpoint is called the vertex.

this implies that the term angle is defined from the undefined term line.

for two dimension i.e., a plane which is a two dimensional figure with infinite length, infinite width and no height.

from the undefined term plane the term angle cannot be defined

therefore, the undefined term which is used to define the term an angle is line.