Cosine -- from Wolfram MathWorld
https://mathworld.wolfram.com/Cosine.html
WebCosine. Download Wolfram Notebook. The cosine function is one of the basic functions encountered in trigonometry (the others being the cosecant, cotangent , secant, sine, and tangent ). Let be an angle measured counterclockwise from the x -axis along the arc of the unit circle . Then is the horizontal coordinate of the arc endpoint.
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