My Math Forum find length if angle and length is given...

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July 18th, 2012, 10:40 PM   #1
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find length if angle and length is given...

Hai all friends..
I got a problem to calculate length of AB...
From figure that I attached.. The question ask me to find the value of R1=AB...
Any one can help me. Thanks...
Attached Images
 2012-07-19 14.39.26.jpg (244.5 KB, 106 views)

 July 19th, 2012, 03:07 AM #2 Senior Member   Joined: Sep 2009 From: Wisconsin, USA Posts: 227 Thanks: 0 Re: find length if angle and length is given... Looking at this I don't think there is enough information, although I could be wrong. It seems to me that we need one more piece of information since $\angle CDB$ could be anything and isn't locked in place. Once we know that angle this problem can easily be solved with vectors. Knowing $\angle CAB$ would also solve the problem.
July 19th, 2012, 04:44 AM   #3
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Re: find length if angle and length is given...

Quote:
 Originally Posted by The_Fool Looking at this I don't think there is enough information, although I could be wrong. It seems to me that we need one more piece of information since $\angle CDB$ could be anything and isn't locked in place. Once we know that angle this problem can easily be solved with vectors. Knowing $\angle CAB$ would also solve the problem.
Ok, now I think I miss 1 item.. that angle CDB[/latex] is 25.3 degree... so how to solve it? tq

 July 19th, 2012, 12:38 PM #4 Global Moderator   Joined: Dec 2006 Posts: 20,484 Thanks: 2041 Let E be the point where a right angle is shown and let F be the foot of the perpendicular drawn from B to DE. By finding DE and DF, calculate EF. By finding CE and BF, calculate AC + CE - BF. Now use Pythagoras.
 July 19th, 2012, 01:08 PM #5 Senior Member   Joined: Sep 2009 From: Wisconsin, USA Posts: 227 Thanks: 0 Re: find length if angle and length is given... My vector solution: As you can see there are already arrows on all line segments in your picture. Also, if you start at one point, you'll end up at the same point after following the series of vectors. The missing angle you specified will allow you to find the solution. So, $\vec{AC}+\vec{CD}+\vec{DB}+\vec{BA}=0$ You then solve for $\vec{BA}$. Then, $R_{\small 1}=\|\vec{BA}\|$.

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