Let
A
B
C
D
be a
cyclic quadrilateral
, where
A
B
=
a
,
B
C
=
b
,
C
D
=
c
,
D
A
=
d
and
a
<
b
<
c
<
d
. Let
∠
B
C
A
=
∠
B
D
A
=
α
,
∠
B
A
C
=
∠
B
D
C
=
β
,
∠
A
B
D
=
∠
A
C
D
=
γ
and
∠
C
A
D
=
∠
C
B
D
=
θ
.
Let the equation x 4 − 2 4 x 3 + 2 0 1 x 2 − 6 9 8 x + 8 4 4 = 0 has roots a , b , c and d .
If the value of sin α + sin θ + sin γ + sin β can be written as n m , where m and n are positive integers with n -square-free, find m + n .
Note: Such quadrilateral indeed exists, the image is up to scale.
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@Alan Enrique Ontiveros Salazar What was strange about this cyclic quadrilateral though?
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Maybe not strange about a particular cyclic quadrilateral, but the fact that such an explicit expression in terms of the coefficients of the 4 th degree polynomial for the answer even exists, which I had guessed had to be the case from the beginning. Working out the details had me thinking for some time. Kudos for Mark Henning's clear exposition of the solution.
I think this is a truly a classic problem.
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Both the numerator and the denominator inside the square root of the expression for R are symmetric polynomials in a , b , c , d . As such (this is standard theory), they must be expressible in terms of the elementary symmetric polynomials a + b + c + d , a b + a c + a d + b c + b d + c d , a b c + a b d + a c d + b c d and a b c d . Calculating the details is a matter of determination!
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@Mark Hennings – At first I thought maybe it worked for just that polynomial in particular, and then later realized it's more like what you just said. How else can such a "neat" value for the solution be accounted for? I can't recall the last time I encountered such a connection between the coefficients of a 4 t h degree polynomial with the value of a trigonometric expression in relation to a geometrical problem. Very interesting.
I agree. Took me a day to realise that the question can actually be solved without finding the roots explicitly. My approach was slightly different (not shorter though).
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@A Former Brilliant Member – I have to admit that at first, I said, "Is this even real?" Took me a while to confirm that it was.
It was just the title of the problem, no special reason.
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By the (extended) Sine Rule, applied to appropriate triangles (all of which have the same circumcircle as the cyclic quadrilateral):
sin α a = sin β b = sin θ c = sin γ d = 2 R
where R is the radius of the circumcircle of the cyclic quadrilateral. Thus X = sin α + sin β + sin γ + sin θ = 2 R a + b + c + d Now a , b , c , d are the zeros of the polynomial f ( X ) = X 4 − 2 4 X 3 + 2 0 1 X 2 − 6 9 8 X + 8 4 4 , and hence a + b + c + d = 2 4 , so that X = R 1 2 .
The circumradius of a cyclic quadrilateral with sides a , b , c , d is
R = 4 1 ( s − a ) ( s − b ) ( s − c ) ( s − d ) ( a b + c d ) ( a c + b d ) ( a d + b c )
where s = 2 1 ( a + b + c + d ) is the semiperimeter. But a 2 + b 2 + c 2 + d 2 a 2 b 2 c 2 + a 2 b 2 d 2 + a 2 c 2 d 2 + b 2 c 2 d 2 ( a b + c d ) ( a c + b d ) ( a d + b c ) ( s − a ) ( s − b ) ( s − c ) ( s − d ) = = = = = = = ( a + b + c + d ) 2 − 2 ( a b + a c + a d + b c + b d + c d ) 2 4 2 − 2 × 2 0 1 = 1 7 4 ( a b c + a b d + a c d + b c d ) 2 − 2 a b c d ( a b + a c + a d + b c + b d + c d ) 6 9 8 2 − 2 × 8 4 4 × 2 0 1 = 1 4 7 9 1 6 a b c d ( a 2 + b 2 + c 2 + d 2 ) + ( a 2 b 2 c 2 + a 2 b 2 d 2 + a 2 c 2 d 2 + b 2 c 2 d 2 ) 8 4 4 × 1 7 4 + 1 4 7 9 1 6 = 2 9 4 7 7 2 f ( s ) = f ( 1 2 ) = 6 7 6 and hence R = 4 1 6 7 6 2 9 4 7 7 2 = 5 2 7 3 6 9 3 and hence X = 7 3 6 9 3 1 2 × 5 2 = 7 3 6 9 3 6 2 4 making the answer 7 4 3 1 7 .