(I already know the unit circle)

Why is it that the sine of an obtuse angle is the sine of its supplementary angle but the cosine of an obtuse angle is the negative of the cosine of its supplementary angle?

I can see of course on the unit circle that it is this way, but my question is:

Is it purely convention? And if it is, what was the logic behind coming up with that specific convention as opposed to another one? Was it just practical to define the unit circle this way?

Why not have the cosine of obtuse angles be defined the same as the sine of obtuse angles? Why would that not work?

  • $\begingroup$ Don't you see that these are the most natural extensions of $\cos$ and $\sin$ to angles outside of $[0,{\pi\over2}]$? Why are you so stubborn? $\endgroup$ Apr 30, 2015 at 18:52
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    $\begingroup$ This answer may be helpful. $\endgroup$
    – Blue
    Apr 30, 2015 at 19:22
  • $\begingroup$ @Blue now that's you have made me understand the OP $\endgroup$ Apr 30, 2015 at 19:24
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    $\begingroup$ @ChristianBlatter, I'm not being stubborn, I'm sorry it comes off this way. I'm in general confused about the line in mathematics between definitions by convenience and definition that follow an actual phenomenon (maybe there is no such line). I think that exploring the limits of definitions and the reasons behind them is important for understanding anything $\endgroup$ Apr 30, 2015 at 19:25
  • $\begingroup$ @Blue, Yes! Thank you...that's exactly the kind of answer I was looking for; it's very helpful for me to see the evolution in the development of definitions. $\endgroup$ Apr 30, 2015 at 19:51

2 Answers 2


Those aren't definitions; they're just facts about how $\cos$ and $\sin$ behave in the second quadrant. Those features follow from how the trig functions are defined using the unit circle: If $(x,y)$ is the point on the unit circle obtained by rotating counterclockwise by $\theta$ radians from the point $(1, 0)$, then $\cos(\theta) := x$ and $\sin(\theta) := y$.

These definitions are built specifically to agree with the "SOH CAH TOA" definitions for $\theta$ between $0$ and $\pi/2$; because the hypotenuse is 1, by definition, the sine of an angle is simply equal to the opposite side, which is $y$, and the cosine of an angle is equal to the adjacent side, which is $x$.

For $\theta$ in the interval from $\pi/2$ to $\pi$ (i.e., $\theta$ an obtuse angle), $(x, y)$ is in the second quadrant, where $x<0$ and $y>0$. Reflecting across the $y$-axis corresponds to taking the supplementary angle, and this reflection negates $x$ and leaves $y$ unchanged. Hence the cosine of $\theta$ is given by the negative of the cosine of the supplementary angle to $\theta$, and the sine is equal to the sine of the supplementary angle.

Since the supplementary angle is given by $\pi-\theta$, these two facts can be summarized by the equations $$\cos(\pi-\theta) = -\cos(\theta)$$ and $$\sin(\pi-\theta)=\sin(\theta),$$ both of which are special cases of the more general angle addition and subtraction formulas.

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    $\begingroup$ As I mentioned in my latest edit, the definition is built to agree with the SOH CAH TOA definition that I assume you're more familiar with. If you move the unit circle to the right, you'll affect the values of cosine and sine for acute angles. $\endgroup$ Apr 30, 2015 at 19:02
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    $\begingroup$ @jeremyradcliff if you think to move unit circle towards right on the x-axis then do you have another plan to define sine and cosine for acute angles? $\endgroup$ Apr 30, 2015 at 19:12
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    $\begingroup$ Would you like to chat? That might be easy... $\endgroup$ Apr 30, 2015 at 19:19
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    $\begingroup$ It sounds like you want to define your own version of sine and cosine, say let's call them $\sin_j$ and $\cos_j$, which would be related to the standard ones via $\sin_j(\theta) = |\sin(\theta)|$ and $\cos_j(\theta)=|\cos(\theta)|$. These are certainly well-defined functions that fit what you seem to desire, but if you graph $\cos_j(\theta)$ as a function of $\theta$, you'll notice a sharp bounce (a corner) in the graph at the point $\pi/2$, whereas with the standard definition it just continues smoothly through. $\endgroup$ Apr 30, 2015 at 19:20
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    $\begingroup$ The standard definitions ensure that $\sin$ and $\cos$ are analytic functions. $\endgroup$ Apr 30, 2015 at 19:21


If I didn't get you wrong

We have a Fundamental Law:


We make deductions and get new identities:

Putting $\alpha=0$ we have,


  • $\begingroup$ So is it correct to think that the development of the unit circle itself stems from the need to have the Trig laws about obtuse angles be coherent with the Fundamental Law you mention? $\endgroup$ Apr 30, 2015 at 18:37
  • $\begingroup$ I hardly understood bit of your comment and so my answer to that little understanding is "Yes" $\endgroup$ Apr 30, 2015 at 19:07
  • $\begingroup$ I would love to chat :) but I'm not sure how, I've never done it before. $\endgroup$ Apr 30, 2015 at 19:33
  • $\begingroup$ join us at chat.stackexchange.com/rooms/36/mathematics $\endgroup$ Apr 30, 2015 at 19:35

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