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$$
z_{3}=2^{{-7/6}}a_{1}^{{1/3}}[f_{Q}({\mathcal  {K}})]^{{1/2}}{\biggl \{}{\biggl [}8^{{1/2}}[f_{Q}({\mathcal  {K}})]^{{-3/2}}-1{\biggr ]}^{{1/2}}-1{\biggr \}}.
$$
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