= Angle between intersecting regular surfaces
{title2=$\theta=\arccos\frac{|\nabla f\cdot\nabla g|}{|\nabla f||\nabla g|}$}
For smooth level surfaces $f=0$, $g=0$ meeting at a point where both <gradients> are nonzero, their acute angle is the <angle between two planes> tangent to the surfaces there. Each gradient is a <normal vector> to its tangent plane, proving the displayed formula. Vanishing gradients require further analysis because the level-set equation then need not specify a unique tangent plane.
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