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INTRODUCTION.
xv
this plainly appears from reasoning, similar to that just employed in defining, or rather describing a point and a line.
The proposition which we have selected to elucidate the manner in which the principles are applied, is the fifth of the first Book.
In an isosceles triangle ABC, the internal angles at the base ABC, ACB are equal, and when the sides AB, AC are produced, the external angles at the base BCE, CBD are also equal.
Produce 
and
make 
=
Draw 
and
(B. 1. pr. 3.)
in 
and 
we have 
=
= and 
common:
∴ = . 
= 
and 
= 
(B. 1. pr. 4.)
Again in 
and 
,
= ,
