Answer:
See the proof below.
Step-by-step explanation:
What we need to proof is this: "Assuming X a vector space over a scalar field C. Let X= {x1,x2,....,xn} a set of vectors in X, where
. If the set X is linearly dependent if and only if at least one of the vectors in X can be written as a linear combination of the other vectors"
Proof
Since we have a if and only if w need to proof the statement on the two possible ways.
If X is linearly dependent, then a vector is a linear combination
We suppose the set
is linearly dependent, so then by definition we have scalars
in C such that:
![c_1 x_1 +c_2 x_2 +.....+c_n x_n =0](https://tex.z-dn.net/?f=%20c_1%20x_1%20%2Bc_2%20x_2%20%2B.....%2Bc_n%20x_n%20%3D0)
And not all the scalars
are equal to 0.
Since at least one constant is non zero we can assume for example that
, and we have this:
![c_1 v_1 = -c_2 v_2 -c_3 v_3 -.... -c_n v_n](https://tex.z-dn.net/?f=%20c_1%20v_1%20%3D%20-c_2%20v_2%20-c_3%20v_3%20-....%20-c_n%20v_n%20)
We can divide by c1 since we assume that
and we have this:
![v_1= -\frac{c_2}{c_1} v_2 -\frac{c_3}{c_1} v_3 - .....- \frac{c_n}{c_1} v_n](https://tex.z-dn.net/?f=%20v_1%3D%20-%5Cfrac%7Bc_2%7D%7Bc_1%7D%20v_2%20-%5Cfrac%7Bc_3%7D%7Bc_1%7D%20v_3%20-%20.....-%20%5Cfrac%7Bc_n%7D%7Bc_1%7D%20v_n%20)
And as we can see the vector
can be written a a linear combination of the remaining vectors
. We select v1 but we can select any vector and we get the same result.
If a vector is a linear combination, then X is linearly dependent
We assume on this case that X is a linear combination of the remaining vectors, as on the last part we can assume that we select
and we have this:
![v_1 = c_2 v_2 + c_3 v_3 +...+c_n v_n](https://tex.z-dn.net/?f=%20v_1%20%3D%20c_2%20v_2%20%2B%20c_3%20v_3%20%2B...%2Bc_n%20v_n)
For scalars defined
in C. So then we have this:
![v_1 -c_2 v_2 -c_3 v_3 - ....-c_n v_n =0](https://tex.z-dn.net/?f=%20v_1%20-c_2%20v_2%20-c_3%20v_3%20-%20....-c_n%20v_n%20%3D0)
So then we can conclude that the set X is linearly dependent.
And that complet the proof for this case.