Trigonometric EquationHard
Question
A tetrahedron has vertices at O (0, 0, 0), A (1, 2, 1), B (2, 1, 3) and C (- 1, 1, 2). Then the angle between the faces OAB and ABC will be
Options
A.cos-1 
B.cos-1 
C.30o
D.90o
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