ELEMENTS OF GEOMETRY. INTRODUCTION. 1. QUANTITY is anything that can be measured. To measure a thing is to find out how many times it contains some other thing of the same kind, taken as 8 standard. The assumed standard is called the unit of mea sure. 2. Since the unit of measure is of the same kind as the thing measured, there are as many species of quantity as there are kinds of units. In Geometry, there are four kinds of units, viz. Units of Length, Units of Surface, Units of Volume, and Units of Angular Measure; and consequently, four species of quantity, viz. LINES, SURFACES, VOLUMES, and ANGLES. These are called GEOMETRICAL MAGNITUDES. 3. GEOMETRY is that branch of Mathematics which treats of the properties and relations of Geometrical Magnitudes. 4. In Geometry, the quantities considered are generally represented by pictorial symbols. The operations to be performed upon them, and the relations between them, are indicated by signs, as in Analysis. The following are the principal signs employed : The Sign of Addition, + +, called plus : Thus, A+ B, indicates that The Sign of Subtraction, B is to be added to A. called minus : Thus, A – B, indicates that B is to be subtracted from A. Thus, A × B, indicates that A is to be multiplied by B. The Sign of Division, A Thus, A÷B, or, B' indicates that A is to be divided by B. The Exponential Sign: Thus, A3, indicates that A is to be taken three times as a factor, or raised to the third power. The Radical Sign, ✓ 3 Thus, √A, B, indicate that the square root of A, and the cube root of B, are to be taken. or When a compound quantity is to be operated upon as a single quantity, its parts are connected by a vinculum by a parenthesis: and C, indi Thus, A+B x C, indicates that the sum of A B is to be multiplied by C; and (A + B) ÷ C, cates that the sum of A and B is to be divided by C. A number written before a quantity, shows how many times it is to be taken. Thus, 3(A+B), indicates that the sum of A and B is to be taken three times. The Sign of Equality, B+C, indicates that A is equal to the Thus, A = B + C, sum of B and C. = The expression, A B+ C, is called an equation. The part on the left of the sign of equality, is called the first member; that on the right, the second member. The Sign of Inequality, <: 3 Thus, √A</B, indicates that the square root of A is less than the cube root of B. The opening of the sign is towards the greater quantity. The sign, ... . is used as an abbreviation of the word hence, or consequently. 5. The general truths of Geometry are deduced by a course of logical reasoning, the premises being definitions and principles previously established. The course of reasoning employed in establishing any truth or principle, is called a demonstration. 6. A THEOREM is a truth requiring demonstration. 8. A PROBLEM is a question requiring a solution. 9. A POSTULATE is a problem whose solution is selfevident. Theorems, Axioms, Problems, and Postulates, are all called Propositions. 10. A LEMMA is an auxiliary proposition. 11. A COROLLARY is an obvious consequence of one more propositions. or 12. A SCHOLIUM is a remark made upon one or more propositions, with reference to their connection, their use, their extent, or their limitation. 13. An HYPOTHESIS is a supposition made, either in the statement of a proposition, or in the course of a demonstration. 14. Two magnitudes are equal, when they are equal in measure. When they may be so placed as to coincide throughout their whole extent, they are equal in all their parts. ELEMENTS OF GEOMETRY. BOOK I. ELEMENTARY PRINCIPLES. DEFINITIONS. 1. GEOMETRY is that branch of Mathematics which treats of the properties and relations of Geometrical Magnitudes. tude. A POINT is that which has position, but not magni 3. A LINE is that which has length, but neither breadth nor thickness. Lines are divided into two classes, straight and curved. 4. A STRAIGHT LINE is one which does not change its direction at any point. 5. A CURVED LINE is one which changes its direction at every point. The word line, alone, is used for straight line; and the word curve, alone, for curved line. 6. A line made up of straight lines, is called a broken line. 7. A SURFACE is that which has length and breadth, without thickness. |