Re: Question in Geometry

On Sep 25, 10:04 am, riderofgiraffes <mathforum.org...@xxxxxxxxxxxxxx>
wrote:
you're not providing any evidence to us
that you are thinking about this at all.
... V1 is of the form [a, 0, 0] where a > 0.

Good.  So what is the normalised form of V1?
What does it mean to be normalised?  Can you
find a vector of length 1 that points in the
same direction as V1?

I also think that the angles between V1 and
V2 is independent of their length,

Correct.

I dont know how to calculate the angle given
only phi and theta and V2.

You've been told that already.  Check again,
and check carefully.  Then if you still have
a problem, tell us what you've tried.

You might also tell us where the problem comes
from so we can better help you understand the
stuff that actually matters.

Alright then, let me rephraise the problem
because I think that I had  made a mistake.

Well, take any vector V1 (in 3D) that is localised
somewhere in the XY plane. In contrast to I wrote
ealier, I dont know anything else about V1 besides
this fact.
Consider the vector V2 with the following properties:
1. The angle between V2 and its projection
   on the XY is Beta.
2. The angle between its projection and the
   vector V1 is Alpha

Now, what is the angle between V1 and V2 where
all I know is: Alpha, Beta and V2.

Well, firstly, you can assume that V1 is just (a,0,0)
because you can rotate the whole thing to align V1
with the X-axis.  Your angles alpha and beta won't
change, and neither does the angle between V1 and V2.

Now you can choose the length of V1, because that
doesn't affect anything either.  You may as well
assume that V1 is simply (1,0,0).

Now the very first answer you got is enough to solve
the problem.  You've still given no indication that
you've read that and tried to use it.  If you're
simply looking for us to give you a formula then
you'll either be very lucky, or you'll have to give
us a reason for simply giving an answer, because you
won't learn anything that way.  In a sense, you'd be
asking us to do your work for you, for free.

Most people here are happy to teach, but we're not
happy to do someone else's work for no reward.

So, given that first answer, what don't you understand?- Hide quoted text -

- Show quoted text -

Given the first answer everything is clear. I only have to rotate V2
using Alpha and Beta towards V1 to obtain its direction and now I can
project one on the other to achieve the angle between them.

Thanks a lot, that is all I have needed.

Now, regarding the other stuff you wrote, I'm sorry that I sound like
what you were implying on me.
Its not that, I only thought that there is a closed simple formula ...

Anyway, thanks again.

.

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