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TRIGONOMETRY: FORMULAS, IDENTITIES, SOLUTION OF TRIANGLES

Trigonometric functions of an acute angle. The basic definitions of the various trigonometric functions are given in terms of the acute angles of a right triangle. See Fig. 1. Shown is a right triangle in which C is the right angle, the side opposite being the hypotenuse c. In terms of this right triangle of Fig. 1 the definitions are as follows:

Trigonometric functions of complementary angles. The acute angles A and
B of the right triangle ABC are complementary, that is A + B = 90^{o}. From Fig. 1 we have

These relations associate the functions in pairs — sine and cosine, tangent and cotangent, secant and cosecant — each function of a pair being called the cofunction of the other. Thus, any function of an acute angle is equal to the corresponding cofunction of the complementary angle.

Trigonometric functions of a general angle. In the above definitions, the
trigonometric functions are only defined for angles between 0^{o} and 90^{o}. For angles outside this
range they are undefined. Using the above definition, the concepts make no sense for angles
outside this range. For many of the applications of trigonometry, as in the problems of
surveying, where the object is in the solution of triangles, the above definitions are probably
adequate. However, the applications of trigonometric concepts are broad and diverse, cropping
up in many fields, especially in physics and engineering, often as part of algebraically expressed
relationships between various variables in the form of equations. For functions appearing in
equations it is highly desirous that the function be defined for all values of the independent
variable, not just some small range of values. Consequently, the definition of the trigonometric
functions has been broadened in such a way as to define meanings for the functions for all angles,
not just the angles between 0^{o} and 90^{o}. The broadened definition utilizes the rectangular
coordinate system shown in Fig. 2. Let P(x, y) be a point on a circle of radius r and let θ be the
angle measured counterclockwise from the positive x axis to line OP. Then the trigonometric
functions are defined as follows:

The coordinate system is divided into four quadrants — quadrants I, II, III, and IV --- as shown in the figure. The values of x are positive to the right of the origin and negative to the left of it, as usual, and the values of y are positive above the origin and negative below it. The signs of the functions change from quadrant to quadrant, depending on the signs of the x and y coordinates of point P. The angle θ can be either positive or negative, with the counterclockwise direction considered as positive, and is generally measured in either degrees or radians. Angles can be of any size — point P can make multiple trips around the origin.

It will be noted that for angles in the first quadrant the functions have the same values as in the previous definition.

Reciprocal relations. An immediate consequence of the definitions above are the following reciprocal relations:

sin θ = 1/csc θ tan θ = 1/cot θ sec θ =1/cosθ

cos θ = 1/sec θ cot θ = 1/tan θ csc θ = 1/sinθ

Spiegel. Mathematical Handbook of Formulas and Tables. pp. 12 -17

CRC Standard Mathematical Tables. pp. 159 - 161

References.

Spiegel. Mathematical Handbook of Formulas and Tables. pp. 12 -17

CRC Standard Mathematical Tables. pp. 159 - 161

Ayres. Trigonometry. (Schaum)

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