(b) Given, $\sec{z = \frac{x^{4} + y^{4} - 8x^{2}y^{2}}{x^{2} + y^{2}}}$ Here, $n = 2$ $\therefore x\frac{\partial z}{\partial x} + y\frac{\partial z}{\partial y} = 2.\frac{\sec z}{\sec{z.\tan z}} = 2\cot z$
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Ph.D. in Physics with 19+ years of teaching experience.