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A complete period of sin θ

THE LINE VALUE of a trigonometric function is a straight line whose length represents the value of the function. For as the central angle changes, the line value becomes a kind of "graph" of the function during that change. It is the best way for the student to visualize and remember each function's values, in particular those of the sine, cosine, and tangent.

The line values

The line values of the trigonometric functions

Let A be the center of a unit circle with radii AD, AB.

From B, drop a perpendicular BC onto AD.

Draw radius AF perpendicular to DA; draw DE, FG tangent to the circle; that is, draw them at right angles to the radii;  (Theorem 13)

and extend AB to meet those tangents at E, G respectively.

Then, since AG is a straight line that crosses the parallel straight lines FG, AD, the alternate angles FGA, GAD are equal.  (Theorem 8)

Therefore, triangles ACB, ADE, GFA are equiangular and therefore similar.  (Theorem 15)

The line values of the functions of angle θ are now as follows; to avoid scrolling, the figure below is the same:

The line values of the trigonometric functions
sin θ   =   CB
  =   CB
  =  CB.
cos θ  =   AC
  =   AC
  =  AC.
tan θ  =   DE
  =   DE
  =  DE.
csc θ  =   AG
  =   AG
  =  AG.
sec θ  =   AE
  =   AE
  =  AE.
cot θ  =   FG
  =   FG
  =  FG.

We now see why the tangent function (DE) is called that:  It is in fact a tangent.  The secant function (AE) is a secant.  It cuts the circle; from the Latin secare, to cut.

A complete period of sin θ

The student should now try to visualize what happens to CB, the line value of sin θ, as AB makes a complete revolution about the unit circle.

Line values

At θ = 0°, the point B coincides with D, so that CB vanishes. sin 0° = 0.

As θ increases in the first quadrant, CB also increases until it reaches its maximum value at 90°.  CB coincides there with the unit radius AF, so that CB = AF = sin 90° = 1.

When θ, angle DAB, enters the second quadrant (Fig. 2), CB begins to decrease in value.  At 180°, B coincides with K, and CB vanishes again. sin 180° = 0.

In the third quadrant (Fig. 3), CB increases, but the sine in that quadrant is algebraically negative.  At 270°, CB coincides with AL. sin 270° has its algebraic minimum:  sin 270° = −1.

Finally, in the fourth quadrant (Fig. 4), CB decreases until B coincides once again with D.  sin 360° = sin 0° = 0.

Upon competing one full circuit, sin θ has gone through every possible value. We say that sin θ has completed one period.  It is customary to use radian measure and say that the period of sin θ is 2π.

That means that every 2π radians, Line values of sine thetasin θ returns to the same line value CB.

For any angle θ, sin (θ + 2π) = sin θ.

We will go into this again in the following topic.

Line values of sine theta

We see moreover that sin θ -- the line value CB -- never has a value greater than 1: at 90°, or less than −1: at 270°.

−1 ≤ sin θ ≤ 1.

A complete period of cos θ

The line value of cos θ is AC.  As the line AB revolves, Line values of cosine theta C shuttles back and forth along the diameter KD. At 0°, AC coincides with the unit radius AD, so that cos 0° = 1.

At θ = 90°, C coincides with A, so that AC vanishes.  cos 90° = 0.

At θ = 180°, C reaches K.   AC = AK = cos 180° = −1.

At θ = 270°, C is back at A.   cos 270° = 0.

Finally, at θ = 360°, AC coincides again with the radius AD.  cos 360° = 1.

The period of cos θ is also 2π.

cos θ (AC) also has a maximum value of 1: at 0°, and minimum of −1: at 180°.

−1 ≤ cos θ ≤ 1.

The period of tan θ

The values of tan θ, like the values of all the trigonometric functions, regularly repeat themselves. The values of tan θ repeat every π units (in radians). And so we say that tan θ has period π.

Line values of tan theta

For as θ takes on values from −π/2 to π/2, the line value DE of tan θ goes through all the real numbers from −∞ to ∞.

The graph of a trigonometric function is essentially its line value plotted as the y-coördinate versus radian angle x. See the following Topic: Graphs of the trigonometric functions.



The straight line CD is called the vertical projection of the straight line AB.  (The straight lines BD, AC are drawn perpendicular to CD.  It is as if a light were shining on AB, and CD is the shadow.)

vertical projection

EF is called the horizontal projection of AB.

Now, if AB is a unit radius, then BC -- the line value of sin θ -- is the

horizontal projection

vertical projection of the radius.  And AC -- the line value of cos θ -- is the horizontal projection.

Thus in a unit circle, sin θ is the vertical projection of the radius.  cos θ is the horizontal projection.

Next Topic:  Graphs of the trigonometric functions

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