T
H
E

E
L
E
M
E
N
T
S
Book I   Proposition 14
But
    the angles CBA, ABD are also equal to two right angles;
therefore
    the angles CBA, ABE are equal to the angles CBA, ABD. [Post.4 and C.N.1]

Let
    the angle CBA be subtracted form each;
therefore
    the remaining angle ABE is equal to the remaining angle ABD, [C.N.3]
the less to the greater: which is impossible.

Therefore BE is not in a straight line with CB.
Previous Page    Return to Propositions    Next Page