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A tetrahedron has vertices P(1, 2, 1), Q(2, 1, 3), R(–1, 1, 2) and O(0, 0, 0). The angle between the faces OPQ and PQR is :

1. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − 31 17 cos 1
2. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − 35 9 cos 1
3. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − 35 19 cos 1
4. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − 31 7 cos 1

Correct answer: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − 35 19 cos 1

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