Page:First six books of the elements of Euclid 1847 Byrne.djvu/39

Rh 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.