# [Tutor] Maths: getting degrees from radians (or am I wrong?)

Bernard Lebel 3dbernard at gmail.com
Wed Sep 21 16:05:38 CEST 2005

```Okay thanks a lot everyone.

Bernard

On 9/20/05, Kent Johnson <kent37 at tds.net> wrote:
> Bernard Lebel wrote:
> > Hello,
> >
> > I have this little math problem. I have this formula from wich I get a
> > dot product between two vectors.
> >
> > cos(ß) = A.B / |A|.|B| = -0.0634
> > So this would give me radians, right?
>
> No, it's the cosine of ß, which has no units (a cosine is a ratio of two lengths)
>
> >
> > Then if I use
> >
> > math.degrees( -0.0634 )
> >
> > This gives me a value of -3.6325524211294193.
> >
> > However I have a book in front of me who says I should get a value of
> > 93.635 degrees. mmmmm
> >
> > Btw, in the book, the equation is written
> >
> > ß = cos-1(-0.0634) = 93.635, where -1 is actually an exponent. Maybe
> > I'm just interpreting this wrong?
>
> The -1 means inverse. You have cos(ß) = -0.0634 - you want to find the angle whose cosine is -0.0634, i.e. (inverse cosine)(-0.0634). Another name for cos-1 is arccosine. In Python it is math.acos():
>  >>> import math
>  >>> math.acos(-0.0634)
> 1.6342388771557625
>  >>> math.degrees(_)
> 93.634990377223801
>
> Kent
> >
> >
> >
> > Thanks
> > Bernard
> > _______________________________________________
> > Tutor maillist  -  Tutor at python.org
> > http://mail.python.org/mailman/listinfo/tutor
> >
> >
>
> _______________________________________________
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>
```