# Linear Transformation: find dilating/rotation matrix

## Homework Statement

The vector A has length 8.5, and makes an angle of 5pi/19 with the x-axis.

The vector B has length 6, and makes an angle of 8pi/19 with the x-axis.

Find the matrix which rotates and dilates vector into vector .

## Homework Equations

Rotation matrix in counterclockwise direction: \begin{equation}

\left[
\begin{array}{ccc}
cos∅ & -sin∅\\
sin∅ & cos∅\\
\end{array}
\right]

\end{equation}

Dilation matrix:
\begin{equation}

\left[
\begin{array}{ccc}
c & 0\\
0 & c\\
\end{array}
\right]

\end{equation}

## The Attempt at a Solution

\begin{equation}

\left[
\begin{array}{ccc}
(12/17)cos(3pi/19) & -sin(3pi/19)\\
sin(3pi/19) & (12/17)cos(3pi/19)\\

\end{array}
\right]

\end{equation}

My line of thinking: to move A into B, it needs to move counterclockwise from its current positions by an angle of 3pi/19. A needs to "shrink" by a factor of 12/17.

However, this is incorrect. I would really appreciate it if someone could point me in the right direction, or even give me a similar example. Thank you.

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Ray Vickson
Homework Helper
Dearly Missed

## Homework Statement

The vector A has length 8.5, and makes an angle of 5pi/19 with the x-axis.

The vector B has length 6, and makes an angle of 8pi/19 with the x-axis.

Find the matrix which rotates and dilates vector into vector .

## Homework Equations

Rotation matrix in counterclockwise direction: \begin{equation}

\left[
\begin{array}{ccc}
cos∅ & -sin∅\\
sin∅ & cos∅\\
\end{array}
\right]

\end{equation}

Dilation matrix:
\begin{equation}

\left[
\begin{array}{ccc}
c & 0\\
0 & c\\
\end{array}
\right]

\end{equation}

## The Attempt at a Solution

\begin{equation}

\left[
\begin{array}{ccc}
(12/17)cos(3pi/19) & -sin(3pi/19)\\
sin(3pi/19) & (12/17)cos(3pi/19)\\

\end{array}
\right]

\end{equation}

My line of thinking: to move A into B, it needs to move counterclockwise from its current positions by an angle of 3pi/19. A needs to "shrink" by a factor of 12/17.

However, this is incorrect. I would really appreciate it if someone could point me in the right direction, or even give me a similar example. Thank you.
You can either rotate first, then shrink, or shrink first, then rotate. Also, be careful about rotations: for rotation a coordinate system through angle θ but leaving a vector v = <x,y> unchanged, the coordinates x' and y' of v in the new coordinate system are given by
$$\pmatrix{x' \\ y'} = \pmatrix{\cos(\theta) & -\sin(\theta)\\ \sin(\theta) & \cos(\theta)} \pmatrix{x\\y}.$$
However, if we rotate the vector v through angle θ but keeping the coordinate system unchanged, the coordinates x' and y' of the new vector v' are given by
$$\pmatrix{x' \\ y'} = \pmatrix{\cos(\theta) & \sin(\theta)\\ -\sin(\theta) & \cos(\theta)} \pmatrix{x\\y}.$$
Note that the two transformation matrices are transposes of each other.

RGV

vela
Staff Emeritus
Homework Helper
The way you combined the rotation and dilation isn't correct. As Ray said, do one first and then the other.

To dilate:\begin{equation}

\left[
\begin{array}{ccc}
(12/17) & 0\\
0 & (12/17) \\

\end{array}
\right]

\end{equation}

To rotate:
\begin{equation}

\left[
\begin{array}{ccc}
cos(3pi/19) & -sin(3pi/19)\\
sin(3pi/19) & cos(3pi/19)\\

\end{array}
\right]

\end{equation}

Does the above look right?

So I'm trying to find matrix, a 2X2 matrix so that: \begin{equation}

\left[
\begin{array}{ccc}
A\\
\\

\end{array}
\right]

*

\left[
\begin{array}{ccc}
8.5cos(5pi/19)\\
8.5sin(5pi/19)\\

\end{array}
\right]

=

\left[
\begin{array}{ccc}
6cos(8pi/19)\\
6sin(8pi/19)\\

\end{array}
\right]

\end{equation}

Is there a way to put these two transformations together, into one matrix, A? Thank you for the help you have given so far.

LCKurtz
Homework Helper
Gold Member
So I'm trying to find matrix, a 2X2 matrix so that: \begin{equation}

\left[
\begin{array}{ccc}
A\\
\\

\end{array}
\right]

*

\left[
\begin{array}{ccc}
8.5cos(5pi/19)\\
8.5sin(5pi/19)\\

\end{array}
\right]

=

\left[
\begin{array}{ccc}
6cos(8pi/19)\\
6sin(8pi/19)\\

\end{array}
\right]

\end{equation}
In your original post, if you multiply your dilation matrix by your rotation matrix correctly, you should have a 12/17 times all four entries. Then try that for your A.

Thank you LCKurts, vela, and Ray! So to summarize: think of the transformations separately, and then multiply the two transformation matrices together.
Thanks again