*412 what value(s) of h is b in plane spanned

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Discussion Overview

The discussion revolves around determining the value(s) of \( h \) for which the vector \( b \) lies in the plane spanned by the vectors \( a_1 \) and \( a_2 \). The context includes mathematical reasoning and linear algebra concepts related to linear combinations and augmented matrices.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant poses the question of what value(s) of \( h \) would place \( b \) in the plane spanned by \( a_1 \) and \( a_2 \), suggesting that \( b \) must be a linear combination of \( a_1 \) and \( a_2 \).
  • A hint is provided that \( b \) can be expressed as \( b = v a_1 + w a_2 \) for some constants \( v \) and \( w \).
  • Another participant constructs the augmented matrix corresponding to the linear combination and performs row reduction to find \( h \), arriving at the conclusion that \( h = 3 \) along with values for \( v \) and \( w \).
  • A correction is made regarding the signs of \( v \) and \( w \), suggesting that they should be \( -7 \) and \( -2 \), respectively, while still agreeing on \( h = 3 \).
  • One participant notes that the original question only asked for \( h \), implying that the additional information about \( v \) and \( w \) may not have been necessary.

Areas of Agreement / Disagreement

Participants generally agree that \( h = 3 \) is the value that allows \( b \) to be in the plane spanned by \( a_1 \) and \( a_2 \). However, there is a disagreement regarding the signs of \( v \) and \( w \>, with one participant suggesting they are \( -7 \) and \( -2 \).

Contextual Notes

The discussion involves assumptions related to linear combinations and the properties of augmented matrices, but these assumptions are not explicitly stated or resolved.

karush
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For what value(s) of $h$ is b in the plane spanned by $a_1$ and $a_2$
$$a_1=\left[\begin{array}{r} 1\\3\\ -1 \end{array}\right],
a_2=\left[\begin{array}{r} -5\\-8\\2 \end{array}\right],
b =\left[\begin{array}{r} 3\\-5\\ \color{red}{h} \end{array}\right]$$

ok this should be obvious but I don't see it..
 
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karush said:
For what value(s) of $h$ is b in the plane spanned by $a_1$ and $a_2$
$$a_1=\left[\begin{array}{r} 1\\3\\ -1 \end{array}\right],
a_2=\left[\begin{array}{r} -5\\-8\\2 \end{array}\right],
b =\left[\begin{array}{r} 3\\-5\\ \color{red}{h} \end{array}\right]$$

ok this should be obvious but I don't see it..
Hint: If b is in the plane formed by a_1 and a_2 then it has to be a linear combination of a_1 and a_2. ie. [math]b = v a_1 + w a_2[/math] for some v, w constants.

Can you finish?

-Dan
 
topsquark said:
Hint: If b is in the plane formed by a_1 and a_2 then it has to be a linear combination of a_1 and a_2. ie. [math]b = v a_1 + w a_2[/math] for some v, w constants.

Can you finish?

-Dan

$\left[\begin{array}{r} 1\\3\\ -1 \end{array}\right]v+
\left[\begin{array}{r} -5\\-8\\2 \end{array}\right]w
=\left[\begin{array}{r} 3\\-5\\ \color{red}{h} \end{array}\right]$
so then the augmented matrix would be
$\left[\begin{array}{rr|r}1 & -5 & 3 \\ 3 & -8 & -5 \\ -1 & 2 & h \end{array}\right]$
then RREF
$\left[ \begin{array}{cc|c} 1 & 0 & -7 \\0 & 1 & -2 \\ 0 & 0 & h - 3 \end{array} \right]$
so $h=3$ following would be $v=7$ and $w=2$

hopefully...
 
Last edited:
karush said:
$\left[\begin{array}{r} 1\\3\\ -1 \end{array}\right]v+
\left[\begin{array}{r} -5\\-8\\2 \end{array}\right]w
=\left[\begin{array}{r} 3\\-5\\ \color{red}{h} \end{array}\right]$
so then the augmented matrix would be
$\left[\begin{array}{rr|r}1 & -5 & 3 \\ 3 & -8 & -5 \\ -1 & 2 & h \end{array}\right]$
then RREF
$\left[ \begin{array}{cc|c} 1 & 0 & -7 \\0 & 1 & -2 \\ 0 & 0 & h - 3 \end{array} \right]$
so $h=3$ following would be $v=7$ and $w=2$

hopefully...
I didn't go looking for it but somehow you are off by a sign. v = -7 and w = -2 and h = 3 is the solution.

-Dan
 
ok I see

however the OP only asked for h

mahalo
 
karush said:
ok I see

however the OP only asked for h

mahalo
I know. It was just an FYI.

-Dan
 

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