# vectors

• Jul 6th 2010, 10:48 AM
rabih2011
vectors
how to get, two vectors of length 26 units and slope 5/12?
• Jul 6th 2010, 10:58 AM
earboth
Quote:

Originally Posted by rabih2011
how to get, two vectors of length 26 units and slope 5/12?

I assume that your vectors are in $\mathbb{R}^2$.

1. Then $\vec {v_1} = (26,0)$

2. The 2nd vector could be $\vec{v_2}=k\cdot (12,5)$

3. Determine k such that $|\vec {v_2}| = 26$

Spoiler:
You should come out with k = 2
• Jul 6th 2010, 01:38 PM
HallsofIvy
Quote:

Originally Posted by rabih2011
how to get, two vectors of length 26 units and slope 5/12?

Call the vector <x, y>. Saying it has length 26 means that $\sqrt{x^2+ y^2}= 26$. Saying that it has slope 5/12 means that x/y= 12. That gives you two equations to solve for x and y. From x/y= 12, x= 12y so the first equation becomes $\sqrt{144y^2+ y^2}= \sqrt{145}y= 26$. You may assume that x and y are positive: if x and y are both positive or both negative, the sign disappears in both formulas. That is, if you find <x, y> which satisfies those equations, so does <-x, -y>. (I don't think it is necessary to "assume" your vectors are in $\mathbb{R}^2$- if they were not they would not have "slope".)
• Jul 6th 2010, 02:10 PM
Soroban
Hello, rabih2011!

Quote:

Find two vectors of length 26 units and slope $\frac{5}{12}$

Let the vector be $\langle x,y\rangle$

Its length is 26: . $\sqrt{x^2+y^2} \:=\:26 \quad\Rightarrow\quad x^2 + y^2 \:=\:676$ .[1]

Its slope is $\frac{5}{12}:\;\;\frac{y}{x} \:=\:\frac{5}{12} \quad\Rightarrow\quad y \:=\:\frac{5}{12}x$ .[2]

Substitute [2] into [1]: . $x^2 + \left(\frac{5}{12}x\right)^2 \:=\:676 \quad\Rightarrow\quad x^2 + \frac{25}{144}x^2 \:=\:676$

. . $\frac{169}{144}x^2 \:=\:676 \quad\Rightarrow\quad x^2 \:=\:576 \quad\Rightarrow\quad x \:=\:\pm24$

Substitute into [1]: . $y \:=\:\frac{5}{12}(\pm24) \:=\:\pm10$

The two vectors are: . $\begin{array}{c}\langle 24,\;10\rangle \\ \langle \text{-}24,\text{-}10\rangle \end{array}$
• Jul 6th 2010, 05:42 PM
HallsofIvy
Arghh! I forgot to square the 5/12! Thanks, Soroban.