$x = {e^\theta }\left( {\theta + \frac{1}{\theta }} \right),y = {e^{ - \theta }}\left( {\theta - \frac{1}{\theta }} \right)$
$x = {e^\theta }\left( {\theta + \frac{1}{\theta }} \right),y = {e^{ - \theta }}\left( {\theta - \frac{1}{\theta }} \right)$
Official Solution
and $y = {e^{ - \theta }}\left( {\theta - \frac{1}{\theta }} \right)$
therefore,$\frac{{dx}}{{d\theta }} = \frac{d}{{d\theta }}\left[ {{e^\theta } \cdot \left( {\theta + \frac{1}{\theta }} \right)} \right]$
$= {e^\theta } \cdot \frac{d}{{d\theta }}\left( {\theta + \frac{1}{\theta }} \right) + \left( {\theta + \frac{1}{\theta }} \right) \cdot \frac{d}{{d\theta }}{e^\theta }$
$= {e^\theta }\left( {1 - \frac{1}{{{\theta ^2}}}} \right) + \left( {\theta + \frac{1}{\theta }} \right){e^\theta }$
$= {e^\theta }\left( {1 - \frac{1}{{{\theta ^2}}} + \theta + \frac{1}{\theta }} \right)$
$= {e^\theta }\left( {\frac{{{\theta ^2} - 1 + {\theta ^3} + \theta }}{{{\theta ^2}}}} \right)$ ……(i)
and $\frac{{dy}}{{d\theta }} = \frac{d}{{d\theta }}\left[ {{e^{ - \theta }} \cdot \left( {\theta - \frac{1}{\theta }} \right)} \right]$
$= {e^{ - \theta }} \cdot \frac{d}{{d\theta }}\left( {\theta - \frac{1}{\theta }} \right) + \frac{d}{{d\theta }}{e^{ - \theta }}\left( {\theta - \frac{1}{\theta }} \right)$
$= {e^{ - \theta }}\left( {1 + \frac{1}{{{\theta ^2}}}} \right) + \left( {\theta - \frac{1}{\theta }} \right){e^{ - \theta }} \cdot \frac{d}{{d\theta }}( - \theta )$
$= {e^{ - \theta }}\left[ {\frac{{{\theta ^2} + 1}}{{{\theta ^2}}} - \frac{{{\theta ^2} - 1}}{\theta }} \right] = {e^{ - \theta }}\left[ {\frac{{{\theta ^2} + 1 - {\theta ^3} + \theta }}{{{\theta ^2}}}} \right]$ ……(ii)
therefore,$\frac{{dy}}{{dx}} = \frac{{dy/d\theta }}{{dx/d\theta }} = \frac{{{e^{ - \theta }}\left( {\frac{{{\theta ^2} + 1 - {\theta ^3} + \theta }}{{{\theta ^2}}}} \right)}}{{{e^\theta }\left( {\frac{{{\theta ^2} - 1 + {\theta ^3} + \theta }}{{{\theta ^2}}}} \right)}}$
$= {e^{ - 2\theta }}\left( {\frac{{ - {\theta ^3} + {\theta ^2} + \theta + 1}}{{{\theta ^3} + {\theta ^2} + \theta - 1}}} \right)$
$\Rightarrow \frac{dy}{dx} = {e^{ - 2\theta }}\left( {\frac{{ - {\theta ^3} + {\theta ^2} + \theta + 1}}{{{\theta ^3} + {\theta ^2} + \theta - 1}}} \right)$
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