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 [MNR 1994; UPSEAT 2000; AIEEE 2003]

\[{{\cos }^{-1}}\left( \frac{17}{31} \right)\]

\[{{\cos }^{-1}}\left( \frac{19}{35} \right)\]

A tetrahedron has vertices at \[O(0,\,0,\,0)\], \[A(1,\,2,\,1),B(2,\,1

1.	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 [MNR 1994; UPSEAT 2000; AIEEE 2003]
A.	\[{{\cos }^{-1}}\left( \frac{19}{35} \right)\]
B.	\[{{\cos }^{-1}}\left( \frac{17}{31} \right)\]
C.	\[30{}^\circ \]
D.	\[90{}^\circ \]
Answer» B. \[{{\cos }^{-1}}\left( \frac{17}{31} \right)\]

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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 [MNR 1994; UPSEAT 2000; AIEEE 2003]

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