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BLACKIE'S ELEMENTARY TEXT-BOOKS.

ELEMENTARY TEXT-BOOK

OF

TRIGONOMETRY.

BY

R. H. PINKERTON, B.A. Oxon.
Late George A. Clark Fellow in Mathematics and Natural Philosophy in the University

of Glasgow; formerly Snell Exhibitioner of Balliol College, Oxford.

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LONDON:
BLACKIE & SON, 49 & 50 OLD BAILEY, E.C.;

GLASGOW, EDINBURGH, AND DUBLIN.

18381.1.2

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PREFACE.

I HAVE endeavoured in this Elementary Text-book to lead the student by easy gradations from the Measurement of Angles to the Solution of Triangles.

Chapter I. is the result of an attempt to treat the subject of Measurement of Angles in a systematic manner. In this chapter I have substituted the terms radian and radian measure, which are now coming into use, for the terms unit of circular measure and circular measure respectively.

The work contains a large number of Examples, which have been selected, for the most part, om papers set in the Science and Art, the Civil Service, and the British University Examinations. Examples involving numerical calculations have been freely introduced. With reference to approximate numerical calculations, it may be well to remind the student of the usual rule, which has been followed throughout the work:- Increase by unity the figure in the last decimal place to which the calculation is to be carried, if the next decimal figure would be 5 or greater than 5.

The sets of Examples marked A, B, C, &c., and also the Miscellaneous Examples at the end, may be omitted on a first reading.

It is hoped that this Text-book will be found useful to students preparing for University Pass Examinations, and other Examinations in which Elementary Trigonometry is included.

Any suggestions or corrections will be gratefully received.

R. H. PINKERTON.

HOGGANFIELD, GLASGOW,

2d August, 1884.

TRIGONOMETRY.

CHAPTER I.

MEASUREMENT OF ANGLES.

a

1. Plane Trigonometry is that branch of Mathematics which treats of angles and of the relations between the angles and sides of plane rectilineal figures.

2. Units of Angle.

In measuring any magnitude, we have first to choose some definite magnitude of the same kind as our standard or unit of measurement; then the number of times the magnitude we are measuring contains the unit is the numerical measure of the magnitude. It is evident that the same magnitude will be represented by different numbers when different units are adopted. For example, the distance of a mile will be represented by the number 1 when a mile is the unit of length, by the number 1760 when a yard is the unit of length, by the number 5280 when a foot is the unit of length, and so on. In like manner, the number expressing the magnitude of au angle will depend on the unit of angle. Two units of angle are in general use: the degree and the radian. The degree is the unit of angle adopted in actual observations of angles, and generally in practical applications of trigonometry; while the radian is the unit of angle in common use in the theoretical parts of mathematics.

3. Practical Unit of Angle--the Degree.

The degree is the angle which is the 90th part of a right angle, and is subdivided into minutes and seconds. The minute of angle is the 60th part of a degree, and the second of angle is the 60th part of a minute of angle. One degree, one minute, and one second are written respectively 1°, 1', 1".

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