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Problems
Contests
International Contests
IMO Longlists
1986 IMO Longlists
80
80
Part of
1986 IMO Longlists
Problems
(1)
Find the angle between planes
Source:
8/29/2010
Let
A
B
C
D
ABCD
A
BC
D
be a tetrahedron and
O
O
O
its incenter, and let the line
O
D
OD
O
D
be perpendicular to
A
D
AD
A
D
. Find the angle between the planes
D
O
B
DOB
D
OB
and
D
O
C
.
DOC.
D
OC
.
geometry
3D geometry
tetrahedron
incenter
geometry proposed