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Problems
Contests
National and Regional Contests
Spain Contests
Spain Mathematical Olympiad
1991 Spain Mathematical Olympiad
4
4
Part of
1991 Spain Mathematical Olympiad
Problems
(1)
angle chasing with the incircle
Source: Spanish Mathematical Olympiad 1991 P4
8/1/2018
The incircle of
A
B
C
ABC
A
BC
touches the sides
B
C
,
C
A
,
A
B
BC,CA,AB
BC
,
C
A
,
A
B
at
A
′
,
B
′
,
C
′
A' ,B' ,C'
A
′
,
B
′
,
C
′
respectively. The line
A
′
C
′
A' C'
A
′
C
′
meets the angle bisector of
∠
A
\angle A
∠
A
at
D
D
D
. Find
∠
A
D
C
\angle ADC
∠
A
D
C
.
geometry
incircle
angle