- #1
Kernul
- 211
- 7
My professor did this exercise that I didn't quite get how she went through all of it.
We have a ##U = {(x, y, z, t) : x+y+z+t = 0}## and ##B_{Im(f)} = \left[ \begin{pmatrix}
7 \\
-3 \\
0 \\
0
\end{pmatrix},
\begin{pmatrix}
3 \\
-3 \\
0 \\
0
\end{pmatrix},
\begin{pmatrix}
5 \\
0 \\
1 \\
-5
\end{pmatrix}\right]## with ##dim(Im(f)) = 3##.
The exercise asks the base and dimension of ##Im(f) \cap U## and of ##Im(f) + U##.
Now she starts the exercise doing this:
##x = - y - z - t##
and
##\left\{
\begin{array}{l}
y = a\\
z = b\\
t = c\\
x = - a - b - c
\end{array}
\right.##
After this she writes
##U = \left\{a \begin{pmatrix}
-1 \\
1 \\
0 \\
0
\end{pmatrix} + b \begin{pmatrix}
-1 \\
0 \\
1 \\
0
\end{pmatrix} + c \begin{pmatrix}
-1 \\
0 \\
0 \\
1
\end{pmatrix} : a, b, c \in \mathbb{R}\right\}##
then she writes ##dim(U) = 3## and ##B_U = \left[
\begin{pmatrix}
-1 \\
1 \\
0 \\
0
\end{pmatrix}, \begin{pmatrix}
-1 \\
0 \\
1 \\
0
\end{pmatrix}, \begin{pmatrix}
-1 \\
0 \\
0 \\
1
\end{pmatrix}\right]##
So here they come my first questions:
1) Why did she put ##x = - y - z - t##?
2) Why did she put everything in a system and put ##y = a, z = b, t = c##?
Then she starts with the intersection saying
##\vec v \in Im(f) \Rightarrow \vec 0##
and then she writes
##\vec v = \alpha \begin{pmatrix}
7 \\
-3 \\
0 \\
0
\end{pmatrix} + \beta \begin{pmatrix}
3 \\
-3 \\
0 \\
0
\end{pmatrix} + \gamma \begin{pmatrix}
5 \\
0 \\
1 \\
-5
\end{pmatrix} = \begin{pmatrix}
7\alpha + 3\beta + 5\gamma \\
-3\alpha - 3\beta \\
\gamma \\
-5\gamma
\end{pmatrix}##
then she put all these in a single equation
##7\alpha + 3\beta + 5\gamma -3\alpha - 3\beta + \gamma -5\gamma = 0##
that becomes ##\gamma = -4\alpha##.
After finding that, she substitutes to all the ##\gamma## in the last matrix and does the operations and she writes
##Im(f) \cap U = \left\{\begin{pmatrix}
- 13\alpha + 3\beta \\
-3\alpha - 3\beta \\
-4\alpha \\
20\alpha
\end{pmatrix} = \alpha \begin{pmatrix}
-13 \\
-3 \\
-4 \\
20
\end{pmatrix} + \beta \begin{pmatrix}
3 \\
-3 \\
0 \\
0
\end{pmatrix} : \alpha, \beta \in \mathbb{R}\right\}##
then she writes ##dim(Im(f) \cap U) = 2## and using a formula(that I know it's the Graussmann formula) writes ##dim(Im(f) + U) = dim(Im(f)) + dim(U) - dim(Im(f) \cap U) = 3 + 3 - 2 = 4## and then she writes ##Im(f) + U = \mathbb{R}^4##
My questions are:
3) Why did she put the vector ##\vec v## only belonging to ##Im(f)##? Shouldn't it belong to ##U## too in order to know the intersection between the two subspaces?
4) Why didn't she write, in the ##\vec v## equation, the independent vectors of ##U## too?
5) Why at one point she sum all the frist, second, third and fourth rows of the vectors in one matrix? And why after that she put everything in a normal equation?
6) And in the end, what does ##Im(f) + U = \mathbb{R}^4## mean?
Sorry for all these questions but really I can't understand how she did all this. I know how to solve it differently but I want to know why she did like this.
We have a ##U = {(x, y, z, t) : x+y+z+t = 0}## and ##B_{Im(f)} = \left[ \begin{pmatrix}
7 \\
-3 \\
0 \\
0
\end{pmatrix},
\begin{pmatrix}
3 \\
-3 \\
0 \\
0
\end{pmatrix},
\begin{pmatrix}
5 \\
0 \\
1 \\
-5
\end{pmatrix}\right]## with ##dim(Im(f)) = 3##.
The exercise asks the base and dimension of ##Im(f) \cap U## and of ##Im(f) + U##.
Now she starts the exercise doing this:
##x = - y - z - t##
and
##\left\{
\begin{array}{l}
y = a\\
z = b\\
t = c\\
x = - a - b - c
\end{array}
\right.##
After this she writes
##U = \left\{a \begin{pmatrix}
-1 \\
1 \\
0 \\
0
\end{pmatrix} + b \begin{pmatrix}
-1 \\
0 \\
1 \\
0
\end{pmatrix} + c \begin{pmatrix}
-1 \\
0 \\
0 \\
1
\end{pmatrix} : a, b, c \in \mathbb{R}\right\}##
then she writes ##dim(U) = 3## and ##B_U = \left[
\begin{pmatrix}
-1 \\
1 \\
0 \\
0
\end{pmatrix}, \begin{pmatrix}
-1 \\
0 \\
1 \\
0
\end{pmatrix}, \begin{pmatrix}
-1 \\
0 \\
0 \\
1
\end{pmatrix}\right]##
So here they come my first questions:
1) Why did she put ##x = - y - z - t##?
2) Why did she put everything in a system and put ##y = a, z = b, t = c##?
Then she starts with the intersection saying
##\vec v \in Im(f) \Rightarrow \vec 0##
and then she writes
##\vec v = \alpha \begin{pmatrix}
7 \\
-3 \\
0 \\
0
\end{pmatrix} + \beta \begin{pmatrix}
3 \\
-3 \\
0 \\
0
\end{pmatrix} + \gamma \begin{pmatrix}
5 \\
0 \\
1 \\
-5
\end{pmatrix} = \begin{pmatrix}
7\alpha + 3\beta + 5\gamma \\
-3\alpha - 3\beta \\
\gamma \\
-5\gamma
\end{pmatrix}##
then she put all these in a single equation
##7\alpha + 3\beta + 5\gamma -3\alpha - 3\beta + \gamma -5\gamma = 0##
that becomes ##\gamma = -4\alpha##.
After finding that, she substitutes to all the ##\gamma## in the last matrix and does the operations and she writes
##Im(f) \cap U = \left\{\begin{pmatrix}
- 13\alpha + 3\beta \\
-3\alpha - 3\beta \\
-4\alpha \\
20\alpha
\end{pmatrix} = \alpha \begin{pmatrix}
-13 \\
-3 \\
-4 \\
20
\end{pmatrix} + \beta \begin{pmatrix}
3 \\
-3 \\
0 \\
0
\end{pmatrix} : \alpha, \beta \in \mathbb{R}\right\}##
then she writes ##dim(Im(f) \cap U) = 2## and using a formula(that I know it's the Graussmann formula) writes ##dim(Im(f) + U) = dim(Im(f)) + dim(U) - dim(Im(f) \cap U) = 3 + 3 - 2 = 4## and then she writes ##Im(f) + U = \mathbb{R}^4##
My questions are:
3) Why did she put the vector ##\vec v## only belonging to ##Im(f)##? Shouldn't it belong to ##U## too in order to know the intersection between the two subspaces?
4) Why didn't she write, in the ##\vec v## equation, the independent vectors of ##U## too?
5) Why at one point she sum all the frist, second, third and fourth rows of the vectors in one matrix? And why after that she put everything in a normal equation?
6) And in the end, what does ##Im(f) + U = \mathbb{R}^4## mean?
Sorry for all these questions but really I can't understand how she did all this. I know how to solve it differently but I want to know why she did like this.