Page:First six books of the elements of Euclid 1847 Byrne.djvu/47

Rh HEN a straight line standing upon another straight line  makes angles with it; they are either two right angles or together equal to two right angles.



If be $$\perp$$ to  then,

and =  (def. 7.), But if be not $$\perp$$ to ,

draw $$\perp$$ ; (pr. 11.)

+ =  (const.),

= =  +

∴ +  =  +  +  (ax. 2.)

= +  =.

Q.E.D.