H
E
E
L
E
M
E
N
T
S
and
let CE have been cut off equal to CD, [I.3]
and
let the equilateral triangle DEF have been constructed upon DE, [I.1]
and
let FC have been joined.
I say that
the straignt line FC has been drawn at right angles to the given straight line AB from C the given point on it.
