What is "in triangle def?

In triangle DEF refers to a specific triangle with vertices named D, E, and F.

Triangles are three-sided polygons, and each side of a triangle is referred to as a "side." The vertices of a triangle are the points where the sides intersect.

When discussing a triangle, the vertices are typically labeled with capital letters, with the first letter representing the angle opposite the corresponding side. For example, in triangle DEF, vertex D corresponds to side EF, vertex E corresponds to side DF, and vertex F corresponds to side DE.

When referencing a specific triangle, it is important to consider the properties and relationships of the sides and angles within that triangle. By using the notation "in triangle DEF," it specifies that the statement or equation being discussed applies specifically to triangle DEF and not to other triangles.