My Math Forum Calculate the acute angle between the lines with equations

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 August 23rd, 2013, 05:07 PM #1 Member   Joined: Oct 2009 From: honolulu Posts: 52 Thanks: 0 Calculate the acute angle between the lines with equations $r=\left(\ 4 \\ -1\right) + s\left(4 \\ 3\right)$ and $r=\left(2 \\ 4\right) + t\left(\ 1 \\ -1\right)$ using the 2 direction vectors $A=\left( 4 \\ 3\right)$ and $B=\left(\ 1 \\ -1 \right)$ $\frac{A \cdot B}{||A||x||B||}$ I get about $8^\circ$, but on W|F it looks closer to $81^\circ$. Last edited by skipjack; April 30th, 2016 at 03:43 PM.
August 23rd, 2013, 05:48 PM   #2
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Re: Calculate the acute angle between the lines with equatio

Quote:
 Originally Posted by bigwave Calculate the acute angle between the lines with equations $r=\left(\ 4 \\ -1\right) + s\left(4 \\ 3\right)$ and $r=\left(2 \\ 4\right) + t\left(\ 1 \\ -1\right)$ using the 2 direction vectors $A=\left( 4 \\ 3\right)$ and $B=\left(\ 1 \\ -1 \right)$ $\frac{A \cdot B}{||A||x||B||}$ I get about $8^o$ but on W|F it looks closer to $81^o$
I think you accidentally took the arcsin of

$\frac{A \cdot B}{\mid \mid A\mid \mid\;\mid\mid B \mid\mid}$

rather than the arccos.

 August 24th, 2013, 03:59 PM #3 Member   Joined: Oct 2009 From: honolulu Posts: 52 Thanks: 0 Re: Calculate the acute angle between the lines with equatio You're right, I forgot to use $cos^{-1}$ on it.

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# calculate my acute angle

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