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  1. Why does the calculator do a cc (counterclockwise) rotation for positive trig ratios instead of clockwise,

  2. and a clockwise rotation for negative sine & tan instead of cc

  3. and a counterclockwise rotation for negative cos ratios instead of a clockwise

ie. in degree mode

$\cos^{-1}(-5/12)=114.62$

$\sin^{-1}(-5/12)=-24.62$

$\tan^{-1}(-5/12)=-22.61$

Is it maybe picking the value that involves the least amount of computing power? or is it a matter of convention? or am I overlooking something?

N. F. Taussig
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Allan Henriques
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    Conventionally, counter clockwise rotations are described by positive angles. But it looks like your question is more about the ranges of the inverse trigonometric functions. – John Doe Apr 22 '19 at 00:25
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    Try using Mathjax: Surround your formulas with $ signs, use \ before a trig function, and {} between the start and end of a superscript. E.g. $\cos^{-1}(-5/12)=114.62$ – wlad Apr 22 '19 at 00:28
  • This tutorial explains how to typeset mathematics on this site. – N. F. Taussig Apr 22 '19 at 01:13

1 Answers1

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This actually has to do with the way inverse trig functions are defined. For a function to be invertible there must be one input for every output. Graphically, this is equivalent to passing the horizontal line test. Now, trig functions are periodic and as such are very much not invertible. The way we get around this is to restrict the domain of each function to a region that passes the horizontal line test.

For $\sin(x)$ the region that we take is $-\frac{\pi}{2}\leq x \leq \frac{\pi}{2}$, or $-90^{\circ} \leq x \leq 90^{\circ}$ in degree mode, as seen in the following plot:

enter image description here

For $\cos(x)$ the region we take is $0\leq x \leq \pi$, or $0^{\circ} \leq x \leq 180^{\circ}$ in degree mode. Note that we could also have taken $-\pi \leq x \leq 0$, but for convenience we take $x$ to be a positive angle.

enter image description here

Lastly, for $\tan(x)$ we can take a full period around the origin, so $-\frac{\pi}{2} \leq x \leq \frac{\pi}{2}$, or $-90^{\circ} \leq x \leq 90^{\circ}$ in degree mode.

enter image description here

DMcMor
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    That makes complete sense! When you see the graph of the functions, sure enough they give out the reasons why the calculators give out the answers they do. – bjcolby15 Apr 22 '19 at 00:51