T
H
E

E
L
E
M
E
N
T
S
Book IV   Proposition 5
Then, since
    AD is equal to DB, and DF is common and at right angles,
therefore
    the base AF is equal to the base FB. [I.4]

Similarly we can prove that
    CF is also equal to AF;

so that
    FB is also equal to FC;
therefore
    the three straight lines FA, FB, FC are equal to one another.
Previous Page    Return to Propositions    Next Page