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dihedral angle

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\begin{document}
Two distinct half-planes, emanating from a same line $l$, \PMlinkescapetext{divide} the space ($\mathbb{R}^3$) into two regions called {\em dihedral angles}.\, The line $l$ is the {\em edge} of the dihedral angle and the bounding half-planes are its {\em sides}.

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The angle, which the sides of a dihedral planes separate from a normal plane of the edge, is the {\em normal section} of the dihedral angle.\, Apparently, all normal sections are equal.\, According to the \PMlinkescapetext{size} of the normal section, the dihedral angle may be called acute, right, obtuse, straight, \PMlinkname{skew}{ConvexAngle}, convex and concave.\, Unlike the angle between two planes, a dihedral angle may be over 90 \PMlinkescapetext{degrees}.

If two planes intersect each other and if one of the four dihedral angles formed is right, then also the others are right.\, Then we say that the planes are {\em perpendicular} to each other.



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