Hello,
I have this little math problem. I have this formula from wich I get a
dot product between two vectors.
cos() = A.B / |A|.|B| = -0.0634
So this would give me radians, right?
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
= cos-1(-0.0634) = 93.635, where -1 is actually an exponent. Maybe
I'm just interpreting this wrong?
Thanks
Bernard
Tutor maillist - Tutor (AT) python (DOT) org

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() = A.B / |A|.|B| = -0.0634
So this would give me radians, right?

No, it's the cosine of , 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

= 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() = -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 (DOT) org

Tutor maillist - Tutor (AT) python (DOT) org

Bernard Lebel wrote:

import math
math.acos(-0.0634)
1.6342388771557625
math.degrees(_) < in all my time on tutor
I have never noticed
this underscore trick
before
93.634990377223801

That's quite handy.

Alan

Tutor maillist - Tutor (AT) python (DOT) org

John Fouhy wrote:
21/09/05, R. Alan Monroe <amonroe (AT) columbus (DOT) rr.comwrote:

math.degrees(_) < in all my time on tutor
>>
>I have never noticed
>this underscore trick
>before

I'm not a big fan of it, actually. It smells of Perl and those opaque
one-liners that make use of implicit functions implicitely setting
implicit variables

It also only works in *interactive* interpreters. -- Poor Yorick

Tutor maillist - Tutor (AT) python (DOT) org

John Fouhy wrote:
21/09/05, R. Alan Monroe <amonroe (AT) columbus (DOT) rr.comwrote:

math.degrees(_) < in all my time on tutor
>>
>I have never noticed
>this underscore trick
>before

I'm not a big fan of it, actually. It smells of Perl and those opaque
one-liners that make use of implicit functions implicitely setting
implicit variables

I'm actually not much of a fan either - it's too easy to lose what I wanted by doing another step, then it's too late. And I usually don't use it on tutor because I think it is a bit obscure. This time I wondered if anyone would notice :-)

Kent

Tutor maillist - Tutor (AT) python (DOT) org

thanks a lot everyone.

Bernard

9/20/05, Kent Johnson <kent37 (AT) tds (DOT) netwrote:
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() = A.B / |A|.|B| = -0.0634
So this would give me radians, right?

No, it's the cosine of , 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

= 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() = -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 (DOT) org

>
>
>

Tutor maillist - Tutor (AT) python (DOT) org

Tutor maillist - Tutor (AT) python (DOT) org