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BOOK I. PROP. V. THEOR.
5
N any isosceles triangle 
if the equal sides be produced, the external angles at the base are equal, and the internal angles at the base are also equal.
Produce 
, and (post. 2.),
take 
= , (pr. 3.);
draw 
and .
Then in 
and 
we have, 
= 
(const.), 
common to both, and = (hyp.) ∴ 
= 
, = and 
= 
(pr. 4.).
Again in 
and 
, we have = , ∴ 
= 
and 
= 
(pr. 4.) but = , ∴ 
= 
(ax. 3.)
Q.E.D.
