Angle between Two Planes 面面角

Given that angle BAC is 40 degrees. Where should be points B and C such that the angle between the two planes is also 40 degrees?[br]已知角BAC固定為40度。移動B點和C點到甚麼位置，兩平面的交角才是40度？[br]
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Information: Angle between Two Planes 面面角