Let a plane α is perpendicular to a plane β, a crosssection of α and β is line a, a line b is in the plane β and the b is perpendicular to the a, then the line b is perpendicular to the plane α. That is, the st. line b is in β, one of the two mutually perpendicular planes, and perpendicular to a, their common section, is perpendicular to α, another plane.
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