Difference between revisions of "Trigonometry"

Revision as of 19:14, 30 May 2011

Trigonometry seeks to find the lengths of a triangle's sides, given 2 angles and a side. Trigonometry is closely related to analytic geometry.

Basic definitions

Usually we call an angle $\theta$ , read "theta", but $\theta$ is just a variable. We could just as well call it $a$ .

For the following definitions, the "opposite side" is the side opposite of angle $\theta$ , and the "adjacent side" is the side that is part of angle $\theta$ , but is not the hypotenuse.

i.e. If ABC is a right triangle with right angle C, and angle A = $\theta$ , then BC is the "opposite side", AC is the "adjacent side", and AB is the hypotenuse.

Sine

The sine of an angle $\theta$ , abbreviated $\sin \theta$ , is the ratio between the opposite side and the hypotenuse of a triangle. For instance, in the 30-60-90 triangle above, $\sin 30^{\circ}=\frac 12$ .

Cosine

The cosine of an angle $\theta$ , abbreviated $\cos \theta$ , is the ratio between the adjacent side and the hypotenuse of a triangle. For instance, in the 30-60-90 triangle above, $\cos 30^{\circ} =\frac{\sqrt{3}}{2}$ .

Tangent

The tangent of an angle $\theta$ , abbreviated $\tan \theta$ , is the ratio between the opposite side and the adjacent side of a triangle. For instance, in the 30-60-90 triangle above, $\tan 30^{\circ}=\frac{\sqrt{3}}{3}$ . (Note that $\tan \theta=\frac{\sin\theta}{\cos\theta}$ .)

Cosecant

The cosecant of an angle $\theta$ , abbreviated $\csc \theta$ , is the ratio between the hypotenuse and the opposite side of a triangle. For instance, in the 30-60-90 triangle above, $\csc 30^{\circ}=2$ . (Note that $\csc \theta=\frac{1}{\sin \theta}$ .)

Secant

The secant of an angle $\theta$ , abbreviated $\sec \theta$ , is the ratio between the hypotenuse and the adjacent side of a triangle. For instance, in the 30-60-90 triangle above, $\sec 30^{\circ}=\frac{2\sqrt{3}}{3}$ . (Note that $\sec \theta=\frac{1}{\cos \theta}$ .)

Cotangent

The cotangent of an angle $\theta$ , abbreviated $\cot \theta$ , is the ratio between the adjacent side and the opposite side of a triangle. For instance, in the 30-60-90 triangle above, $\cot 30^{\circ}=\sqrt{3}$ . (Note that $\cot \theta=\frac{\cos\theta}{\sin\theta}$ .)

Trigonometery Definitions for non-acute angles

Consider a unit circle that is centered at the origin. By picking a point on the circle, and dropping a perpendicular line to the x-axis, a right triangle is formed with a hypotenuse 1 unit long. Letting the angle at the origin be $\theta$ and the coordinates of the point we picked to be $(x,y)$ , we have:

$\sin \theta = y$

$\cos \theta = x$

$\tan \theta = \frac{y}{x}$

$\csc \theta = \frac{1}{y}$

$\sec \theta = \frac{1}{x}$

$\cot \theta = \frac{x}{y}$

Note that $(x,y)$ is the rectangular coordinates for the point $(1,\theta)$ .

This is true for all angles, even negative angles and angles greater than 360 degrees. Due to the way trig ratios are defined for non-acute angles, the value of a trig ratio could be positive or negative, or even 0.

Trigonometric Identities

There are many identities that are based off of trigonometric functions.

Pythagorean Identities

$\sin^2\theta+\cos^2\theta=1$
$1+\tan^2\theta=\sec^2\theta$
$1+\cot^2\theta=\csc^2\theta$

Double-Angle Identities

$\sin 2\theta=2\sin\theta\cos\theta$
$\cos 2\theta=\cos^2\theta-\sin^2\theta$
$\tan 2\theta=\frac{2\tan\theta}{1-\tan^2\theta}$

Half-Angle Identites

$\sin\frac{\theta}{2}=\pm\sqrt{\frac{1-\cos\theta}{2}}$
$\cos\frac{\theta}{2}=\pm\sqrt{\frac{1+\cos\theta}{2}}$
$\tan\frac{\theta}{2}=\pm\sqrt{\frac{1-\cos\theta}{1+\cos\theta}}$

@@ Line 47: / Line 47: @@
 This is true for all angles, even negative angles and angles greater than 360 degrees. Due to the way trig ratios are defined for non-acute angles, the value of a trig ratio could be positive or negative, or even 0.
+==Trigonometric Identities==
+There are many identities that are based off of trigonometric functions.
+===Pythagorean Identities===
+*<math>\sin^2\theta+\cos^2\theta=1</math>
+*<math>1+\tan^2\theta=\sec^2\theta</math>
+*<math>1+\cot^2\theta=\csc^2\theta</math>
+===Double-Angle Identities===
+*<math>\sin 2\theta=2\sin\theta\cos\theta</math>
+*<math>\cos 2\theta=\cos^2\theta-\sin^2\theta</math>
+*<math>\tan 2\theta=\frac{2\tan\theta}{1-\tan^2\theta}</math>
+===Half-Angle Identites===
+*<math>\sin\frac{\theta}{2}=\pm\sqrt{\frac{1-\cos\theta}{2}}</math>
+*<math>\cos\frac{\theta}{2}=\pm\sqrt{\frac{1+\cos\theta}{2}}</math>
+*<math>\tan\frac{\theta}{2}=\pm\sqrt{\frac{1-\cos\theta}{1+\cos\theta}}</math>
 ==See also==
 * [[Trigonometric identities]]

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