Page:First six books of the elements of Euclid 1847 Byrne.djvu/19

Rh 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 ,

= ,