GEOMETRICAL DEFINITIONS OF PLANE A line is length without breadth. The extremities of a line are points. A straight line is that which lies evenly between its extreme points. A plane surface is that in which any two points, being taken, the straight line between them lies wholly in that surface. The extremities of a surface are lines. A plane rectilineal angle is the inclination of two straight lines to one another in a plane which meet together, but are not in the same straight line as in Fig. 1. When a straight line, standing on another straight line, makes the adjacent angles equal to one another, each of the angles is called a right angle and the straight line which stands on the other is called a perpendicular to it as in Fig. 2. GEOMETRICAL DEFINITIONS OF PLANE A line is length without breadth. A straight line is that which lies evenly between its extreme points. A plane surface is that in which any two points, being taken, the straight line between them lies wholly in that surface. The extremities of a surface are lines. A plane rectilineal angle is the inclination of two straight lines to one another in a plane which meet together, but are not in the same straight line as in Fig. 1. When a straight line, standing on another straight line, makes the adjacent angles equal to one another, each of the angles is called a right angle and the straight line which stands on the other is called a perpendicular to it as in Fig. 2. An obtuse angle is that which is greater than a right angle as in Fig. 3. Fig. 3. An acute angle is that which is less than a right angle as in Fig. 1. A term or boundary is the extremity of anything. An equilateral triangle is that which has three equal sides as in Fig. 4. An isosccles triangle is that which has two sides equal as in Fig. 5. Fig. 6. A scalene triangle is that which has three unequal sides as in Fig. 6. A right angled triangle is that which has a right angle as in Fig. 7. An obtuse-angled triangle is that which has an obtuse angle as in Fig. 6. The hypothenuse in a right-angled triangle is the side opposite the right angle as in Fig. 7. A square is that which has all its sides equal and all its angles right-angled as in Fig. 8. A rectangle is that which has all its angles right angles, but only its opposite sides equal as in Fig. 9. A rhombus is that which has all its sides equal, but its angles are not right angles as in Fig. 10. A quadrilateral figure which has its opposite sides parallel is called a parallelogram as in Figs. 8, 9 and 10. A line joining two opposite angles of a quadrilateral is called a diagonal. |