H
E
E
L
E
M
E
N
T
S
BE is equal to ED, for E is the center and EA is common and at right angles,
therefore
the base AB is equal to the base AD. [I.4]
For the same reason
each of the straight lines BC, CD is also equal to each of the straight lines AB, AD;
therefore
the quadrilateral ABCD is equilateral.
