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# Why is tan^-1(tan 10pi/11) = -pi/11 ?

Posted 10 years ago
 I thought that if the inverse was taken to its function, it would just cancel itself out and leave the value inside. Thus, tan^-1(tan 10pi/11) = 10pi/11, But the correct answer is -pi/11. Can someone please explain to me why? I would greatly appreciate the help. Thank you.
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Posted 10 years ago
 Tan is not invertible as a whole. ArcTan is the inverse of the restriction to the interval between -Pi/2 to Pi/2. Your angle 10Pi/11 is outside that interval. The angle between -Pi/2 and Pi/2 which has the same Tan as 10Pi/11 is precisely -Pi/11. Have a look: With[{\[Theta] = 10 Pi/11}, Graphics[{Circle[], PointSize[Large], Point[{Cos[\[Theta]], Sin[\[Theta]]}], Line[{{1, -1}, {1, 1}}], Line[{{Cos[\[Theta]], Sin[\[Theta]]}, 2 {Cos[\[Theta] - Pi], Sin[\[Theta] - Pi]}}]}, Axes -> True]] 
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