Question: 1 ) For $ mathbf { a } = a _ x mathbf { e } _ x + a _ y

1) For $\mathbf{a}=a_x \mathbf{e}_x+a_y \mathbf{e}_y+a_z \mathbf{e}_z,\mathbf{b}=b_x \mathbf{e}_x+b_y \mathbf{e}_y+b_z \mathbf{e}_z$ find $\mathbf{c}=\mathbf{a}\times \mathbf{b}$ by three methods: term-by-term, resolving the determinant $\mathbf{c}=\left|\begin{array}{lll}\mathbf{e}_x & \mathbf{e}_y & \mathbf{e}_z \\ a_x & a_y & a_z \\ b_x & b_y & b_z\end{array}\right|$, and using the permutation symbol: $\mathbf{c}=\mathbf{a}\times \mathbf{b}=\sum_{i=1}^3\sum_{m=1}^3\sum_{n=1}^3\varepsilon_{i m n} a_m b_n \mathbf{e}_i ; \begin{aligned} & \varepsilon_{123}=\varepsilon_{231}=\varepsilon_{312}=+1\\ & \varepsilon_{321}=\varepsilon_{213}=\varepsilon_{132}=-1\end{aligned}$, all others $=0$
2) For a tensor represented in Cartesian matrix form as $\mathbf{T}=\left|\begin{array}{ccc}T_{x x} & T_{x y} & 0\\ T_{y x} & T_{y y} & 0\\0 & 0 & T_{z z}\end{array}\right|$,
Find the double dot product $\mathbf{T}: \mathbf{T}$.
3) For $\mathbf{v}=v_x \mathbf{e}_x+v_y \mathbf{e}_y+v_z \mathbf{e}_z$, and $\boldsymbol{\tau}=\tau_{x x}(x, y, z)\mathbf{e}_x \mathbf{e}_x+\tau_{x y}(x, y, z)\mathbf{e}_x \mathbf{e}_y \ldots+\tau_{z z}(x, y, z)\mathbf{e}_z \mathbf{e}_z$, find $\mathbf{v}\cdot
abla \boldsymbol{\tau}$.
4)2-D Cartesian coordinate system $\left[\mathbf{e}_x^{\prime},\mathbf{e}_y^{\prime}\right]$ is oriented at angle $\theta=35^{\circ}$, with repect to the horizontal/vertical system $\left[\mathbf{e}_x,\mathbf{e}_y\right]$. We have position vectors $\mathbf{r}=3\mathbf{e}_x+4\mathbf{e}_y$ and $\mathbf{r}^{\prime}=4.752\mathbf{e}_x^{\prime}+1.556\mathbf{e}_y^{\prime}$. Show that the two vectors are equal, $\mathbf{r}^{\prime}=\mathbf{r}$; i.e., that they have the same magnitude and direction. There is a bit of roundoff error.

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