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.

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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.

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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.

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Hexagon.

Heptagon.