Reference Angles

Every angle that is not quadrantal has what is called a reference angle, and that is the acute angle between the terminal side and the x-axis.  A reference angle is always positive and less than 90^o.  The value of a trig function will always be plus-or-minus the value of the function of the reference angle.  More precisely, the absolute value of a trig function of an angle will always equal the value of the trig function of its reference angle:

 

Example: 

If θ = 100^o, then the reference angle is 80^o and:

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