Page:First six books of the elements of Euclid 1847 Byrne.djvu/67

Rh F any side of a triangle be produced, the external angle  is equal to the sum of the two internal and opposite angles ( and ), and the three internal angles of every triangle taken together are equal to two right angles.

Through the point draw || (pr. 31.).

∴  +  =  (ax. 2.),

and therefore

+  +   =   =  (pr. 13.).

Q. E. D.