EXPLANATION OF SIGNS. 



The straight AB means the aggregate of all points situated 

 in the same straight line with A and B. 



The sect AB means that piece of the straight AB between 

 the points A and B. * 



The ray AB means that half of the straight AB which com 

 mences at the point A and contains the point B. 



The plane ABC means the aggregate of all points situated 

 in the same plane as the three points (not in a 

 straight) A, B, C. 



The hemi-plane ABC means that half of the plane ABC 

 which starts from the straight AB and contains the 

 point C. 



ABC means the smaller of the pieces into which the plane 

 ABC is parted by the rays BA, BC, or the non-reflex 

 angle of which the sides are the rays BA, BC. 



ABCD (the point D being situated within / ABC, and the 

 straights BA, CD not intersecting) means the portion 

 of / ABC comprised between ray BA, sect BC, ray 

 CD; while BACD designates the portion of the plane 

 ABC comprised between the straights AB and CD. 



J_ is the sign of perpendicularity. 

 || is the sign of parallelism. 



/ means angle. 



rt. / is right angle. 



st. / is straight angle. 



^ is the sign of congruence, indicating tliat two magni 

 tudes are superposable. 



AB^CD means /CAB=/ACD. 



x~a means x converges toward the limit a. 



A is triangle. 



Qr means the [circumference of the] circle of radius r. 



area Qr means the area of the surface of the circle of radius r. 



