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Alephu5
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Homework Statement
Suppose [itex]\mathbf{u,v,w} \in {\rm I\!R}^2[/itex] are noncollinear points, and let [itex]\mathbf{x} \in {\rm I\!R}^2[/itex].
Show that we can write [itex]\mathbf{x}[/itex] uniquely in the form [itex]\mathbf{x} = r\mathbf{u} + s\mathbf{v} + t\mathbf{w}[/itex], where [itex]r + s + t = 1[/itex].
Homework Equations
Suppose [itex]\mathbf{a,b} \in {\rm I\!R}^2[/itex] are non-parallel.
Where [tex]A = \begin{bmatrix} a_1 & b_1 \\
a_2 & b_2
\end{bmatrix}[/tex]
We know [itex]A^{-1}[/itex] exists because [itex]\mathbf{a}[/itex] and [itex]\mathbf{b}[/itex] are non-parallel.
If [itex]\mathbf{c} = A^{-1}\mathbf{x} = \begin{bmatrix} c_1 \\ c_2\end{bmatrix}[/itex] for some [itex]\mathbf{x} \in {\rm I\!R}^2[/itex] then [itex]\mathbf{x} = A\mathbf{c} = c_1\mathbf{a} + c_2\mathbf{b}[/itex].
Thus any [itex]\mathbf{x}[/itex] can be represented as a unique linear combination of [itex]\mathbf{a}[/itex] and [itex]\mathbf{b}[/itex].
The Attempt at a Solution
[itex]\mathbf{u-w}[/itex] and [itex]\mathbf{v-w}[/itex] must be non-parallel, so let [itex]\mathbf{a} = \mathbf{u-w}[/itex] and [itex]\mathbf{b} = \mathbf{v-w}[/itex] (substituting into the theorem above) so that [tex]\mathbf{x} = c_1\mathbf{u} + c_2\mathbf{v} - (c_1 + c_2)\mathbf{w}.[/tex]
But substituting [itex]r,s [/itex] and [itex]t[/itex] from the hypothesis of the question gives [tex] r + s = -t [/tex]
or
[tex]r + s +t = 0.[/tex]
I have been wondering if perhaps there is an important distinction between 'noncollinear points' and the vectors between the origin and the given points. If we don't allow ourselves to treat the points like vectors, then it only makes sense to talk about linear combinations of subtractions; i.e. [itex]c_1(\mathbf{u-w}) + c_2(\mathbf{v-w})[/itex] but not [itex]c_1\mathbf{u} - c_2\mathbf{w}[/itex] or [itex]c\mathbf{v}[/itex] (what is the scalar multiple of a point?). With this interpretation my approach no longer applies to this problem, but it also so happens that the problem itself does not make any sense.
So by treating the points as heads of vectors I must be working with the authors intended mindset. I have been fiddling with this for over an hour and cannot see what I'm doing wrong. Can anyone help?
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