Suppose there are two planes that have normal vectors n1 and n2. Let vector n2' = -n2. So, n2' is also the normal vector of plane 2. We can see that the angle between n1 and n2, and between n1 and n2' is different. Some say that the angle between two planes is the angle between their normals. If there are two possible angles between their normals, which one is the correct angle between the planes?
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