The Basic Trig Functions
Some Definitions
First let's define the six trig functions:
sine, cosine, tangent, secant, cotangent and cosecant, and see what
they have to do with triangles.
The mnemonic SOH-CAH-TOA captures the definitions of the three most
common trig functions, at least for angles between 0 and $\pi/2$ radians (90 degrees).
$$\text{Sine }=\frac{\text{Opposite}}{\text{Hypotenuse}}$$
$$\displaystyle \text{Cosine }= \frac{\text{Adjacent}}{\text{Hypotenuse}}$$
$$\displaystyle \text{Tangent }= \frac{\text{Opposite}}{\text{Adjacent}}$$
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The other trig functions are defined in terms of first three:
$$\text{Secant }= \frac{1}{\text{Cosine}} =
\frac{\text{Hypotenuse}}{\text{Adjacent}}$$
$$\text{Cosecant }= \frac{1}{\text{Sine}} =
\frac{\text{Hypotenuse}}{\text{Opposite}}$$
$$\text{Cotangent }= \frac{1}{\text{Tangent}} =
\frac{\text{Adjacent}}{\text{Opposite}}$$
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Below is how to find them in the unit circle.
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