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If $\theta_1$ and $\theta_2$ be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip $\theta$ is given by
Explanation
$\cot\theta_1 = \cot\theta/\cos\alpha$ and $\cot\theta_2 = \cot\theta/\sin\alpha$; squaring and adding gives the relation.
Detailed Solution
$\cot^2\theta = \cot^2\theta_1 + \cot^2\theta_2$ (standard relation for apparent dips in two perpendicular vertical planes)