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Ireland National Math Olympiad
2013 Irish Math Olympiad
6
distinct points
distinct points
Source:
July 7, 2013
trigonometry
Problem Statement
The three distinct points
B
,
C
,
D
B, C, D
B
,
C
,
D
are collinear with C between B and D. Another point A not on the line BD is such that
∣
A
B
∣
=
∣
A
C
∣
=
∣
C
D
∣
.
|AB| = |AC| = |CD|.
∣
A
B
∣
=
∣
A
C
∣
=
∣
C
D
∣.
Prove that ∠
B
A
C
=
36
BAC = 36
B
A
C
=
36
if and only if
1
/
∣
C
D
∣
−
1
/
∣
B
D
∣
=
1
/
(
∣
C
D
∣
+
∣
B
D
∣
)
1/|CD|-1/|BD|=1/(|CD| + |BD|)
1/∣
C
D
∣
−
1/∣
B
D
∣
=
1/
(
∣
C
D
∣
+
∣
B
D
∣
)
.
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