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 Calculus Calculus Math Forum

 March 6th, 2016, 08:17 AM #1 Senior Member   Joined: Oct 2014 From: Complex Field Posts: 119 Thanks: 4 Simple vector angle question gone wrong Hello, I don't even know where to put this question because it's quite basic, but I get a different answer than the one in the book: I have 2 vectors, AM and AN: AM=w+0.5u AN=w+0.5v Also, |u|=|v|=|w|=1. What is the angle MAN? I did (AMAN)/(|AM|*|AN|) and got that cos(MAN)=0.8, thus MAN=36.86, but the angle in the book is 25.84, which fits to cos(MAN)=0.9, where is my mistake? Thank you! March 6th, 2016, 08:29 AM #2 Math Team   Joined: Jul 2011 From: Texas Posts: 2,836 Thanks: 1479 I agree with your calculation ... maybe a book typo Thanks from noobinmath March 6th, 2016, 08:46 AM #3 Senior Member   Joined: Oct 2014 From: Complex Field Posts: 119 Thanks: 4 Thank you sir March 6th, 2016, 11:52 AM #4 Newbie   Joined: Jan 2016 From: Ghana Posts: 17 Thanks: 4 Could one of you show your working on how you got the angle? I did it and got a different angle, I think I may have missed a step or something March 6th, 2016, 12:06 PM #5 Math Team   Joined: Jul 2011 From: Texas Posts: 2,836 Thanks: 1479 $\vec{AM} = 0.5u + 0v + w$ $\vec{AN} = 0u + 0.5v + w$ $\vec{AM} \cdot \vec{AN} = (0.5)(0) + (0)(0.5) + (1)(1) = 1$ $|AM| = \sqrt{0.5^2 + 0^2 + 1^2} = \sqrt{1.25}$ $|AN| = \sqrt{0^2 + 0.5^2 + 1^2} = \sqrt{1.25}$ $\cos{\theta} = \dfrac{\vec{AM} \cdot \vec{AN}}{|AM||AN|} = \dfrac{1}{1.25} = 0.8$ $\theta = \arccos(0.8 ) \approx 37^\circ$ Thanks from noobinmath and Lanthanide Tags angle, question, simple, vector, wrong Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post pcompany Probability and Statistics 8 September 30th, 2013 10:37 PM yogazen2013 Algebra 2 August 12th, 2013 01:51 AM Insertnamehere Calculus 4 September 6th, 2009 09:14 PM spa0324 Advanced Statistics 0 May 25th, 2009 10:05 AM Majestic_Q Algebra 1 December 4th, 2008 03:49 PM

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