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

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.