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Class 9 Algebra 2 Questions asked by Filo students

Question 4
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An important part in many applications such as optimization, mechanics, and partial differential equations (PDE) is taking derivatives with respect to vectors. By definition, if \( \boldsymbol{y}=\left[\begin{array}{llll}y_{1} & y_{2} & \ldots & y_{n}\end{array}\right]^{T} \) and \( \boldsymbol{x}=\left[\begin{array}{llll}x_{1} & x_{2} & \ldots & x_{n}\end{array}\right]^{T} \) are two vectors and \( \boldsymbol{y}=f(\boldsymbol{x}) \), which means \( \boldsymbol{y} \) is dependent on \( \boldsymbol{x} \), then \[ \frac{\partial \boldsymbol{y}}{\partial \boldsymbol{x}}=\left[\begin{array}{cccc} \frac{\partial y_{1}}{\partial x_{1}} & \frac{\partial y_{1}}{\partial x_{2}} & \cdots & \frac{\partial y_{1}}{\partial x_{n}} \\ \frac{\partial y_{2}}{\partial x_{1}} & \frac{\partial y_{2}}{\partial x_{2}} & \cdots & \frac{\partial y_{2}}{\partial x_{n}} \\ \vdots & \vdots & \ddots & \vdots \\ \frac{\partial y_{n}}{\partial x_{1}} & \frac{\partial y_{n}}{\partial x_{2}} & \cdots & \frac{\partial y_{n}}{\partial x_{n}} \end{array}\right] . \] 1. Using the definition, determine \( \frac{\partial \boldsymbol{y}}{\partial \boldsymbol{x}} \) if \( \boldsymbol{y}=A \boldsymbol{x} \), when \[ A=\left[\begin{array}{ll} 1 & 2 \\ 3 & 4 \end{array}\right], \quad A=\left[\begin{array}{lll} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \end{array}\right] \] Make a guess: what if \( A \) is a general \( n \times n \) matrix? 2. The definition also holds even when \( \boldsymbol{y} \) is a scalar (a vector of length 1 ). What is \( \frac{\partial \boldsymbol{y}}{\partial \boldsymbol{x}} \), if \( \boldsymbol{y}=\boldsymbol{x}^{T} \boldsymbol{x} \) (Hint: you can try with \( \boldsymbol{x} \) has length 2,3 , and generalize)?
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