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## Precalculus

### Course: Precalculus > Unit 7

Lesson 7: Matrices as transformations of the plane- Matrices as transformations of the plane
- Working with matrices as transformations of the plane
- Intro to determinant notation and computation
- Interpreting determinants in terms of area
- Finding area of figure after transformation using determinant
- Understand matrices as transformations of the plane
- Proof: Matrix determinant gives area of image of unit square under mapping
- Matrices as transformations
- Matrix from visual representation of transformation

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# Finding area of figure after transformation using determinant

In this worked example, Sal finds the area of the image of a rectangle after a transformation defined by a given matrix. This is done by finding the area of the pre-image and multiplying by the matrix's determinant. Created by Sal Khan.

## Want to join the conversation?

- Why do you use the absolute value of determinant here?(2 votes)
- You are multiplying the determinant by the area, and you cannot have a negative area. If you want the shape to be smaller, it would be multiplied by a fraction, such as 1/4, not a negative number.(3 votes)

- I get the way to find the area, it's quite easy. But I don't fully understand why we need to muliply the determinant at0:55. In this situation, what does the determinant tell us? Thanks in advance!(2 votes)

## Video transcript

- [Tutor] We're told to consider
this matrix transformation or this is a matrix that you can view, represents a transformation on the entire coordinate plane. And then they tell us that the
transformation is performed on the following rectangle. So this is the rectangle
before the transformation and they say, what is the area of the image of the rectangle
under this transformation? So the image of the rectangle
is what the rectangle becomes after the transformation. So pause this video and
see if you can answer that before we work through it on our own. All right, so the main
thing to realize is, if we have a matrix transformation or a transformation matrix like this if we take the absolute
value of its determinant, that value tells us how much
that transformation scales up areas of figures. So let's just do that, let's evaluate the absolute
value of the determinant here. So the absolute value of the determinant would be the absolute value of 5 times 8, 5 times 8 minus 9 times 4, 9 times 4. Remember for a 2 by 2 matrix, the determinant is just this times this minus this times that. And so that's going to be the
absolute value of 40 minus 36 which is just the absolute value of 4 which is just going to be equal to 4. So this tells us that this transformation will scale up area by a factor of 4. So what's the area before
the transformation? Well, we can see that this is, let's see, it's 5 units tall and it is 7 units wide. So this has an area of 35 square
units, pre transformation. So post transformation,
we just multiply it by the absolute value of
the determinant to get, let's see, 4 times 30 is 120 plus 4 times 5 is another 20. So this is going to get us to 140 square units and we're done.