October 7th, 2013, 01:21 AM  #1 
Newbie Joined: Aug 2012 Posts: 11 Thanks: 0  vector geometry
[attachment=0:2pmepyt5]Untitled.png[/attachment:2pmepyt5] Please help me solve this 
October 8th, 2013, 03:10 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 21,027 Thanks: 2258 
(a) If ? is the angle between u and v, u·v = uvcos(?). Even though there's a proof in this article, it doesn't quite make sense, because "the angle between" isn't properly defined  is it an acute angle, an obtuse angle, or what? However, the result is correct once the desired angle has been defined appropriately. Are you using a textbook? 
October 8th, 2013, 05:35 PM  #3 
Newbie Joined: Aug 2012 Posts: 11 Thanks: 0  Re: vector geometry
The angle is acute. I am ok with the a) and c) parts. I need help with the b) part only 
October 8th, 2013, 11:21 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 21,027 Thanks: 2258 
For part (a), it isn't satisfactory to insist that the angle is acute, as the dot product of two vectors can be negative (which requires that the angle between the vectors is not always taken to be acute).


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geometry, vector 
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