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Mathematics
Mathematics
Problem
A tetrahedron has vertices
O
(
0
,
0
,
0
)
O(0, 0, 0)
O
(
0
,
0
,
0
)
,
A
(
1
,
2
,
1
)
A(1, 2, 1)
A
(
1
,
2
,
1
)
,
B
(
2
,
1
,
3
)
B(2, 1, 3)
B
(
2
,
1
,
3
)
and
C
(
−
1
,
1
,
2
)
C(-1, 1, 2)
C
(
−
1
,
1
,
2
)
. Then the angle between the faces
O
A
B
OAB
O
A
B
and
A
B
C
ABC
A
B
C
will be a.
cos
−
1
(
19
35
)
\cos^{-1}\left(\frac{19}{35}\right)
cos
−
1
(
35
19
)
b.
cos
−
1
(
17
31
)
\cos^{-1}\left(\frac{17}{31}\right)
cos
−
1
(
31
17
)
c.
30
∘
30^\circ
3
0
∘
d.
90
∘
90^\circ
9
0
∘
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