TRIGONOMETRY - MEASURE OF AN ANGLE

Any real number tex2html_wrap_inline183 may be interpreted as the radian measure of an angle as follows: If tex2html_wrap_inline185 , think of wrapping a length tex2html_wrap_inline183 of string around the standard unit circle C in the plane, with initial point P(1,0), and proceeding counterclockwise around the circle; do the same if tex2html_wrap_inline193 , but wrap the string clockwise around the circle. This process is described in Figure 1 below.

Figure 1

If Q(x,y) is the point on the circle where the string ends, we may think of tex2html_wrap_inline183 as being an angle by associating to it the central angle with vertex O(0,0) and sides passing through the points P and Q. If instead of wrapping a length s of string around the unit circle, we decide to wrap it around a circle of radius R, the angle tex2html_wrap_inline183 (in radians) generated in the process will satisfy the following relation:

  equation34

Observe that the length s of string gives the measure of the angle tex2html_wrap_inline183 only when R=1.

As a matter of common practice and convenience, it is useful to measure angles in degrees, which are defined by partitioning one whole revolution into 360 equal parts, each of which is then called one degree. In this way, one whole revolution around the unit circle measures tex2html_wrap_inline219 radians and also 360 degrees (or tex2html_wrap_inline223 ), that is:

  equation39

Each degree may be further subdivided into 60 parts, called minutes, and in turn each minute may be subdivided into another 60 parts, called seconds:

  align48

EXAMPLE 1 Express the angle tex2html_wrap_inline225 in Degree-Minute-Second (DMS) notation.

Solution: We use Equation 3 to convert a fraction of a degree into minutes and a fraction of a minute into seconds:

align59

Therefore, tex2html_wrap_inline227 .

EXAMPLE 2 Express the angle tex2html_wrap_inline225 in radians.

Solution: From Equation 2 we see that

displaymath179

EXAMPLE 3 Find the length of an arc on a circle of radius 75 inches that spans a central angle of measure tex2html_wrap_inline231 .

Solution: We use Equation 1, tex2html_wrap_inline233 , with R=75 inches and
tex2html_wrap_inline237 , to obtain

displaymath180

Here are some more exercises in the use of the rules given in Equations 1,2, and 3.

EXERCISE 1 Express the angle tex2html_wrap_inline239 radians in (a) decimal form and (b) DMS form.

Solution

EXERCISE 2 Express the angle tex2html_wrap_inline241 in radians.

Solution

EXERCISE 3 Assume that City A and City B are located on the same meridian in the Northern hemisphere and that the earth is a sphere of radius 4000 mi. The latitudes of City A and City B are tex2html_wrap_inline243 and tex2html_wrap_inline245 , respectively.

(a)
Express the latitudes of City A and City B in decimal form.

(b)
Express the latitudes of City A and City B in radian form.

(c)
Find the distance between the two cities.

Solution

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Luis Valdez Sanchez
Wed Dec 4 17:54:59 MST 1996

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