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October 7th, 2013, 01:21 AM   #1
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vector geometry

[attachment=0:2pmepyt5]Untitled.png[/attachment:2pmepyt5]
Please help me solve this
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 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 = |u||v|cos(?). 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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