Page:First six books of the elements of Euclid 1847 Byrne.djvu/81

PON a given ſtrait line () to conſtruƈt a ſquare.

Draw $\perp$ and $=$ (pr. 11. and 3.)

Draw $\parallel$ , and meeting drawn $\parallel$ .

In $=$ (conſt.)

$=$ a right angle (conſt.)

$\therefore$ $=$ $=$ a right angle (pr. 29.),

and the remaining fides and angles muſt be equal, (pr. 34.)

and $\therefore$ is a ſquare. (def. 27.)

Q. E. D