3,52 GEOMETRY. 



A right-angled triangle is one that has in it a 

 right-angle, as A B C, Fig. 7- 



A triangle cannot have more than one right-an- 

 gle. The side opposite to the right-angle B, as 

 AC, is called the hypothenuse, and is always the 

 longest side. 



An obMe-artglcd triangle has one obtuse angle, 

 as Fig. 8. 



An acute-angled triangle has all its angles acute, 

 as Fig. 4. 



An isosceles, or a scalene triangle, may be 

 either right-angled, obtuse, or acute. 



Any side of a triangle is said to subtend the 

 angle opposite to it : thus A B (Fig, 7)> subtends 

 the angle A C B. 



If the side of a triangle be drawn out beyond 

 the figure, as A D (Fig. 8), the angle A, or C A B, 

 is called an internal angle, and the angle CAD, 

 or that without the figure, an external angle. 



17. A figure with four sides is called a quadri- 

 lateral figure. They are of various denominations, 

 as their sides are equal or unequal, or as all their 

 angles are right angles or not. 



18. Every four-sided figure whose opposite sides 

 are parallel, is called a parallelogram. Provided 

 that the sides opposite to each other be parallel, it 

 is immaterial whether the angles are right or not. 

 Fig. 9, 10, 11, and 12, are all parallelograms. 



When the angles of a parallelogram are all 

 right angles, it is called a rectangular parallel- 

 ogram, or a rectangle, as Fig. 11 and 12. 



19. A rectangle may have all its sides equal, or 

 only the opposite sides equal. When all its sides 

 are equal, it is called a square, as Fig. 12. 



20. When the opposite sides are parallel, and all 

 the sides equal to each other, but the angles not 



