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## Linear algebra

### Unit 2: Lesson 1

Functions and linear transformations- A more formal understanding of functions
- Vector transformations
- Linear transformations
- Visualizing linear transformations
- Matrix from visual representation of transformation
- Matrix vector products as linear transformations
- Linear transformations as matrix vector products
- Image of a subset under a transformation
- im(T): Image of a transformation
- Preimage of a set
- Preimage and kernel example
- Sums and scalar multiples of linear transformations
- More on matrix addition and scalar multiplication

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# Matrix from visual representation of transformation

Learn how to determine the transformation matrix that has a given effect that is described visually.

## Warmup example

Let's practice encoding linear transformations as matrices, as described in the previous article. For instance, suppose we want to find a matrix which corresponds with a 90degrees rotation.

The first column of the matrix tells us where the vector $\greenD{\left[ \begin{array}{c} 1 \\ 0 \end{array} \right]}$ goes, and—looking at the animation—we see that this vector lands on $\left[ \begin{array}{c} 0 \\ 1 \end{array} \right]$. Based on this knowledge, we start filling in our matrix like this:

For the second column, we ask where the vector $\redD{\left[ \begin{array}{c} 0 \\ 1 \end{array} \right]}$ lands. Rotating this upward facing vector 90degrees yields a leftward facing arrow—i.e., the vector $\left[ \begin{array}{c} -1 \\ 0 \end{array} \right]$—so we can finish writing our matrix as $\left[ \begin{array}{cc} 0 & \redD{-1} \\ 1 & \redD{0} \end{array} \right]$.

Now you try!

## Practice problems

**Problem 1**

What matrix corresponds with the following transformation?

**Problem 2**

What matrix corresponds with the following transformation?

**Problem 3**

What matrix corresponds with the following transformation?

**Problem 4**

What matrix corresponds with the following transformation?

**Problem 5**

What matrix corresponds with the following transformation?

**Problem 6**

What matrix corresponds with the following transformation?

## Want to join the conversation?

- You lost me on this one.(8 votes)
- The answer to each question can be easily found by looking at where the green and red basis vectors end up after each transformation, and encoding where they do in a 2X2 matrix as follows:
*_ _*

| green-x-coord red-x-coord |

| |

| green-y-coord red-y-coord |

|*_ _*|

It is really that simple. The green vector is the vector that defines the x-axis and has length 1, labeled "i" with a little caret on top, pronounced "eye-hat," while the red arrow in the vector that defines the y-axis and has length 1 as well, labeled "j" with a little caret on top, pronounced "jay-hat." The 2x2 matrix has two columns: the left column says where i-hat ends up after the transformation, and the right column says where j-hat does.

I hope that helps a bit. :-)(8 votes)

- Hi everyone,

after seeing the videos, I understand how to do a transformation that corresponds to a 90° rotation or a rotation multiple of 90°.

Now I am wondering what might be a rule to translate a transformation expressed as a matrix into a rotaion expressed in degree.

My intuition says that

1.) I would need to rotate the x and the y component by the same degree if I do not wish to somehow squeez my vector/figure/coordinate system.

2.) I would probably able to calculate the matrix that corresponds to a rotation of given degrees using the unity circle and the sine etc functions.

So I would need to calculate the angle phi for any given point P in the coordinate system, add the angle I would like to rotate it, and then calculate the respective x,y coordinates.

Is this reasoning correct?

And if I want to resize the object (as in the tasks earlier), I assume, I could alter the magnitude of the vectors pointing to each corner of the object. But what would I need to do to alter the size of object and altering the distance to the origin at the same time, e.g. enlarge a triangle and at the same time get it closer to the origin? Would that still be a linear transformation?

Thanks in advance :-)(6 votes)- Your assumption is correct but you don't need to retrieve the original angle of the rotated point first. The trigonometric identities tell you that x'=x⋅cos(θ)-y⋅sin(θ) and y'=x⋅sin(θ)+y⋅cos(θ). And since matrices are all about forming i values each formed from j others values in different proportions, the matrix come by itself :
`+cos θ -sin θ`

+sin θ +cos θ

Since θ is a given constant, sin θ and cos θ are also constants, so you can replace each every expression in the matrix by its numerical equivalent if you wish to do so.

As regards your last question, to translate your figure, you indeed need to ADD a matrix to your result, which is no longer linear. However, it remains possible to perform an affine transformation in a single operation (multiplication of a square matrix), by adding one more dimension : https://en.wikipedia.org/wiki/Transformation_matrix#Affine_transformations(2 votes)

- So we are saying that the result of every linear transformation of the vector zero is the vector zero? In order to be a Linear Transformation?(3 votes)
- Yes, taking the transformation of the zero vector for any linear transformation always gives back the zero vector in the other space.(7 votes)

- Are matrix transformations always represented as a 2x2 matrix? The textbook I'm using suggests that but I want to be sure.(3 votes)
- No. Any matrix of any size mxn is a matrix transformation.

If you are specifically referring to transformations that apply rotation, projection, reflection, etc. in the xy plane, then yes, those only come from 2x2 matrices.(6 votes)

- Is it standard to use

[1, 0]

[0, 1]

as a base for linear bidimensional transformations? or does this need to be specified like so:

[1, 0] > [a, b]

[0, 1] > [c, d](3 votes)- The columns (not rows) of the n * n identity matrix are considered the standard basis for R^n, so yes, [1, 0]^T and [0, 1]^T are the columns of I_2 and are thus the standard basis for anything in R^2. They even have special names: {i-hat and j-hat}, or {e_1, e_2}.(3 votes)

- Each vector in the question is broken into its corresponding x and y. However, I am wondering that instead of expressing the linear transformation by the matrix, could I express the transformation by using a general formula and without breaking down the vector?

Like in the warmup example, suppose there is a vector-valued function h in which two-dimensional vector V is transformed into two-dimensional vector W. Then I generalize a function h(v1, v2)=(-v2, v1), where in the right part of the equation the -v2 represents the returned w1, and v1 represents the returned w2. This specific function could represent the 90-degree rotation, as desired.

And thus what's the difference between using a linear algebra function and the matrix method which breaks down the vector? Does one have more advantages over the other? Thanks a lot.(3 votes)- A good observation. Any linear function can be represented using a matrix. The benefits of using a matrix is that we get to use all the theory associated with matrices, such as Gaussian elimination, eigenvectors, determinants, matrix multiplication (as opposed to function composition), inverse matrices, etc.(3 votes)

- I honestly think 3b1b's Essence of Linear Algebra is one of the most enlightening things you can find on the internet.(2 votes)
- You are right, but to be honest, combining both Khacacademy's problem sets and 3b1b's video helped me a lot to grasp visualization more deeply.(3 votes)

- Maybe I don't quite catch it, but why in every sample the vector start point is [0,0]?

And what will be if we will operate with more complicated vectors, what the transformation matrix will look like?(2 votes)- Don't forget that linear transformations mean the origin does not move. So pinning the vectors to the origin means we can easily see what happens to them.(2 votes)

- Can someone explain what the vector actually does to the object in question 5? Does it rotate

[-2 0]

[-1 -1]

the camera (or object) 180 degrees left or right, does it increase/decrease the size by -1, what exactly does it do? Thanks!(2 votes)- It's a very odd transformation, to be sure.

It is rotate 180 degrees, scaled by 2 in the x-axis, but also seems to have what is called a "shear" transformation. See: https://en.wikipedia.org/wiki/Shear_mapping .

tl;dr, it is where the graph "shifts" on either side of an axis by +k magnitude (not direction).

See that the (1,0) gets put into (-2, -1).. it would be (-1, 0) due to the rotation, * 2 = (-2, 0) for scaling, and - (0, 1) = (-2, -1) for sheer.

Hope this helps!(3 votes)

- How does Example #6 end up as [1,.5] [.5, 1]? It looks to me like the vector lands on [-1,0], [0,-1]. What am I seeing wrong? Thanks!(1 vote)
- Maybe they have fixed it, but it is currently accepting:

[-1, 0]

[0, -1]

as the correct answer for #6 as you say it should be.(3 votes)