Activity 2.6.3

In this activity, we seek to describe various matrix transformations by finding the matrix that gives the desired transformation. All of the transformations that we study here have the form $T:\real^2\to\real^2$.

1. Find the matrix of the transformation that has no effect on vectors; that is, $T(\xvec) = \xvec$.

2. Find the matrix of the transformation that reflects vectors in $\real^2$ across the line $y=x$.

3. What is the result of composing the reflection you found in the previous part with itself; that is, what is the effect of reflecting across the line $y=x$ and then reflecting across this line again? Provide a geometric explanation for your result as well as an algebraic one obtained by multiplying matrices.

4. Find the matrix that rotates vectors counterclockwise in the plane by $90^\circ$.

5. Compare the result of rotating by $90^\circ$ and then reflecting in the line $y=x$ to the result of first reflecting in $y=x$ and then rotating $90^\circ$.

6. Find the matrix that results from composing a $90^\circ$ rotation with itself four times; that is, if $T$ is the matrix transformation that rotates vectors by $90^\circ$, find the matrix for $T\circ T\circ T \circ T$. Explain why your result makes sense geometrically.

7. Explain why the matrix that rotates vectors counterclockwise by an angle $\theta$ is $$\left[\begin{array}{rr} \cos\theta & -\sin\theta \\ \sin\theta & \cos\theta \\ \end{array}\right]$$.