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Definition: The six functions sine, cosine, tangent, cosecant, secant, and cotangent. These functions can be defined several different ways.
Source: "Precalculus" by David H. Collingwood, K. David Prince, and Matthew M. Conroy at University of Washington and mathwords.com
Example: Let t be a real number. We define the sine function y = sin(t), the cosine function y = cos(t) and the tangent function y = tan(t) by the rules: sin(t) = y-coordinate of P(t) = sin(t radians), cos(t) = x-coordinate of P(t) = cos(t radians), and tan(t) = sin(t) / cos(t) = tan(t radians)