PLANE TRIGONOMETRY. III. DEFINITIONS. 1. Plane Trigonometry treats of the solution of plane triangles. In every plane triangle there are six parts, three angles. When three of these parts are three sides and given, one being a side, the remaining parts may be found by computation. The operation of finding the unknown parts is called the solution of the triangle. 10 2. A plane angle is measured by the arc of a circle included between its sides; the centre of the circle being at the vertex, and its radius being 1. The circle, for convenience, is divided into 360 equal parts called degrees; 90 of these parts are included in a quadrant, which includes one-quarter of the circle, and is the measure of a right angle. Each degree is further divided into 60 equal parts called minutes, and each minute into 60 equal parts called seconds. and seconds are denoted by the symbols о ,, ": thus the expression 7° 22' 33" is read, 7 degrees, 22 minutes, and 33 seconds. 3. The complement of an angle is the difference between that angle and a right angle. C Degrees, minutes, M T N M D P M 4. The supplement of an angle is the difference between that angle and two right angles. 5. Instead of employing the arcs themselves, certain functions of the arcs are usually employed, as explained below. A function of a quantity is something which depends upon that quantity for its value. The sine of an angle is the distance from one extremity of the arc enclosing it, to the diameter, through the other extremity. Thus PM is the sine of the angle M O A. The cosine of an angle is the sine of the complement of the angle. Thus N M = OP is the cosine of the angle M O A. The tangent of an angle is a right line which touches the enclosing arc at one extremity, and is limited by a right line drawn from the centre of the circle through the other extremity: the sloping line which thus limits the tangent is called the secant of the angle. A T is the tangent and OT the secant of the angle MOA. The versed sine of an angle is that part of the diameter AP which is intercepted between the foot of the sine and the extremity of the enclosing arc. The cotangent of an angle is the tangent of the complement of that angle: the co-versed sine and cosecant are similarly defined. Thus BT', BN, and OT are respectively the cotangent, co-versed sine, and cosecant of the angle M O A. These terms are in practice indicated by obvious contractions; as, sin. A for the sine of A, cos. A for the cosine of A, &c. 6. The above definitions have been made with reference to a radius of 1. Any function of an arc whose radius is R is equal to the corresponding function of an arc whose radius is 1, multiplied by the radius R. So also any function of an arc whose radius is 1 is equal to the corresponding function of an arc whose radius is R, divided by that radius. 7. Arcs greater than half a circle are not treated in this book, for the reasons that they seldom occur in field practice, and that any young engineer, conversant with the methods herein given, will, it is presumed, have no difficulty in bandling such exceptional problems. Those curious in trigonometrical research should consult special treatises on the subject. IV. NATURAL SINES, ETC. 1. Natural sines, cosines, tangents, or cotangents are those which are referred to a radius of 1. They may be used for all the purposes of trigonometrical computation; but it is found more convenient, in many cases, to employ a table of logarithmic functions. V. LOGARITHMIC SINES, ETC. 1. Logarithmic functions are the logarithms of the natural functions treated in foregoing Art. IV., the characteristics throughout being transformed by the algebraic addition of 10, which has the effect of referring the tabular functions to a posed, simplifying computation. For the intelligent use of these tables, however, it should be remembered that the characteristic 9 indicates a negative characteristic 1, being one less than 10; a characteristic 7 indicates a negative characteristic - 3, and so on. Hence, in figuring with these logarithms, as many tens must be subtracted from the final result as have been added in the operation. This rule is illustrated in the following examples. TO FIND THE LOGARITHMIC FUNCTIONS OF AN ARC WHICH IS EXPRESSED IN DEGREES AND MINUTES. 2. If the arc is less than 45°, look for the degrees at the top of the page, and for the minutes in the left-hand column; then follow the corresponding horizontal line till you come to the column designated at the top by sine, cosine, tang., or cotang., as the case may be; the number there found is the logarithm sought. 3. If the angle is greater than 45°, look for the degrees at the bottom of the page, and for the minutes in the right-hand column; then follow the corresponding line towards the left, till you come to the column designated at the bottom by sine, cosine, tang, or cotang, as the case may be; the number there found is the logarithm sought. 4. If the arc is expressed in degrees, minutes, and seconds, proceed as before with the degrees and minutes; then multiply the corresponding number taken from column D by the number of seconds, and add the product to the preceding result, for the sine or tangent, and subtract it therefrom for the cosine or cotangent. Example. |