Erecting regular pentagons on the sides of a triangle internally leads to the inner pentagon point, while doing so externally gives the outer pentagon point.
Triangle Pentagon Erecting
See also
Inner Pentagon Point, Outer Pentagon PointExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Triangle Pentagon Erecting." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/TrianglePentagonErecting.html