Translations:Two orthogonal sets of circles/19/en

${\begin{aligned}&{\frac {\rm {1}}{{\rm {sin}}^{\rm {2}}{\theta }_{\rm {1}}}}-{\frac {\rm {1}}{{\rm {tan}}^{\rm {2}}{\theta }_{\rm {1}}}}{\rm {=}}{\rm {cosh}}^{\rm {2}}at-{\rm {sinh}}^{\rm {2}}\ {\textit {at}}.\end{aligned}}$

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