A geometric word that begins with F could be figure.
formula
Geometry
eicosohedron
right.
Tangent.
Whole Number
vector, vertex, vertical
Since this sentence is lacking a predicate, it is virtually impossible to answer. "Geometry words that start with r" is a legitimate noun phrase, but is so incomplete that it cannot be answered.
diameter, diallation
I know one is X-Axis
x axsis
none