45. The radius of a circle is a right line, drawn from the centre to the circumference. 46. The diameter of a circle is a right line, drawn through the centre, and terminating in the circumference on both sides. 47. An arc of a circle is any part of the circumference. 48. A chord is a right line, joining the extremities of an arc. : 49. A segment is any part of a circle, bounded by an arc and its chord. 50. A 37. A trapezium is a quadrilateral, which has not both pair of its opposite sides parallel. 54. In a right-angled triangle, the side opposite to the right angle is called the hypotenuse; and the other two the legs, or sides, or sometimes the base and perpendicular. 38. A trapezoid has only one pair of opposite sides parallel. 1 39. A diagonal is a right line, joining any two opposite angles of a quadrilateral. 40. Plane figures, having more than four sides, are, in general, called polygons; and they receive other particular names, according to the number of their sides or angles. 41. A pentagon is a polygon of five sides; a hexagon has six sides; a heptagon, seven ; an octagon, eight; a nonagon, nine; a decagon, ten; an undecagon, eleven; and a dodecagon, twelve. 42. A regular polygon has all its sides and all its angles equal. If they be not both equal, the polygon is irregular. 43. An equilateral triangle is also a regular figure of three sides, and the square is one of four; the former being also called a trigon, and the latter a tetragon. Pentagon. [ Hexagon. Heptagon. |