The difference between an obtuse angle and a semi-circle, or 180 degrees, is called the supplement of that angle. For example, in Fig. 3, angle C D B is the supplement of the angle A D B because angle C D B, added to angle A C B, is equal to a semi-circle or an angle of 180 degrees. Plane Figures are bounded by straight lines, and are named according to the number of sides which they contain. Thus, the space included within three straight lines, forming three angles, is called trilateral figure or triangle. A Right-angled Triangle has one right angle: The sides forming the right angle are called the base and perpendicular; the side opposite the right angle is named hypothenuse. See Fig. 4, A B C is a right-angled triangle-A B is the base, C B is the perpendicular and A C is the hypotheAn isosceles triangle has only two sides equal; an equilateral triangle has all its sides of equal length. An acute-angled triangle has all its angles acute, and an obtuse-angled triangle has one of its angles only obtuse. nuse. Quadrilateral Figures are literally four-sided figures; they are also called quadrangles, because they have four angles. A Parallelogram is a figure whose opposite sides are parallel, as A B C D, in Fig. 5. A Rectangle is a parallelogram having four right angles, as A B C D, in Fig. 5. A Square is an equilateral rectangle, having all its sides and angles equal, like Fig. 5. A Diagonal is a straight line drawn between D two opposite angular points of a quadrilateral figure, or between any two angular points of a polygon. Should the figure be a parallelogram, the diagonal will divide it into two equal triangles, the opposite sides and angles of which will be equal to one another. Let A B C D, Fig. 6, be a parallelogram; join A C, then A C is a diagonal, and the triangles A D C and A B C, into which it divides the parallelogram, are equal. A Polygon is a portion of a plane terminated on all sides by straight lines. A regular polygon has all its sides and angles equal, and an irregular polygon has its sides and angles unequal. FIG.7. FIG. 8. Polygons are named according to the number of their sides or angles, as follows: FIG. 9. FIG. 10. A Triangle is a polygon of three sides. See Fig. 7. See Fig. 12. Figures having more than twelve sides are generally designated polygons, or many-angled figures. FIG. 13. FIG. 14. A Circle is a plane figure bounded by one uniformly curved line, called the circumference, every part of which is equally distant from a point within, called the center, as A in Fig. 17. оо FIG.15. FIG. 16. The Radius of a Circle is a straight line drawn from the center to the circumference; hence, all the radii of a circle are equal, as AB, AD, AC, AE, in Fig. 17. The Diameter of a Circle is a straight line drawn through the center and terminated on each side by the circumference; consequently the circumference is exactly twice the length of the radius, and, therefore, the radius is sometimes called the semi-diameter. See B A C, Fig. 17. The Chord of an Arc is any straight line drawn from one point in the circumference of a circle to another, joining the extremeties of the arc, and dividing the circle either into two equal or unequal parts. If into two equal parts, the chord is also the diameter, and the space included between the arc and the diameter, on either side of it, is called a semicircle. If the parts cut off by the chord are unequal, each of them is called a segment of a circle; but, unless otherwise stated, it is always understood that the smaller arc or segment is spoken of as in Fig. 18 A B is the chord of the arc A C B. If a straight line be drawn from the center of a circle to meet the chord of an arc perpendicularly, |