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A210250
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Area A of the cyclic quadrilaterals such that A, the sides and the radius of the circumcircle are integers.
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12
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48, 192, 240, 432, 480, 672, 768, 936, 960, 1200, 1440, 1680, 1728, 1920, 2160, 2352, 2640, 2688, 2856, 3072, 3744, 3840, 3864, 3888, 4032, 4320, 4368, 4536, 4800, 5016, 5040, 5376, 5712, 5760, 5808, 5880, 6000, 6048, 6072, 6696, 6720, 6912, 7056, 7392, 7560, 7680, 7728, 7752, 7920
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OFFSET
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1,1
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COMMENTS
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In Euclidean geometry, a cyclic quadrilateral is a quadrilateral whose vertices all lie on a single circle. This circle is called the circumcircle or circumscribed circle, and the vertices are said to be concyclic.
The area A of a cyclic quadrilateral with sides a, b, c, d is given by Brahmagupta's formula: A = sqrt((s - a)(s - b)(s - c)(s - d)) where s, the semiperimeter is s = (a+b+c+d)/2.
The circumradius R (the radius of the circumcircle) is given by:
R = sqrt[(ab+cd)(ac+bd)(ad+bc)]/4A.
The corresponding R of a(n) are not unique; for example, for a(12) = 1680 => (a,b,c,d) = (24, 24, 70, 70) with R = 37 and (a,b,c,d) = (40, 40, 42,42) with R = 29.
The smallest corresponding R of a(n) is {5, 10, 13, 15, 17, 25, 20, 25, 26, 25, 41, 29, ...}.
Properties of this sequence:
A majority of quadrilaterals [a, b, c, d] have the property that a = b and c = d, and in this case s = a+c, A = a*c and R = sqrt(a^2+c^2)/2. Because a and c are even => a = 2p and c = 2q, then A = 4pq and R = sqrt(p^2+q^2). Consequently, 2*A103251(n) is included in this sequence.
Nevertheless, there also exist quadrilaterals whose four sides are distinct, for example [a, b, c, d] = [14, 30, 40, 48] => A = 936 = a(8) and R = 25. The subset of a(n) with this property is {936, 2856, 3744, 3864, 4536, 5016, 5376, 5712, 5880, 6696, 7056, 7560, ...}.
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REFERENCES
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Mohammad K. Azarian, Circumradius and Inradius, Problem S125, Math Horizons, Vol. 15, Issue 4, April 2008, p. 32.
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LINKS
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EXAMPLE
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48 is in the sequence because, for (a,b,c,d) = (6,6,8,8),
s = (6+6+8+8)/2 = 14;
A = sqrt((14-6)(14-6)(14-8)(14-8)) = 48;
R = sqrt((6*6+8*8)(6*8+6*8)(6*8+6*8))/(4*48) = 960/192 = 5.
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MATHEMATICA
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SMax=8000;
Do[
Do[
x=S^2/(u v w);
If[u+v+w+x//OddQ, Continue[]];
If[v+w+x<=u, Continue[]];
r=Sqrt[v w+u x]Sqrt[u w+v x]Sqrt[u v+w x]/(4S);
If[r//IntegerQ//Not, Continue[]];
(*{a, b, c, d}=(u+v+w+x)/2-{u, v, w, x}; {a, b, c, d, r, S}//Sow*);
S//Sow; Break[]; (*to generate a table, comment out this line and uncomment previous line*)
, {u, S^2//Divisors//Select[#, S<=#^2&]&}
, {v, S^2/u//Divisors//Select[#, S^2<=u#^3&&#<=u&]&}
, {w, S^2/(u v)//Divisors//Select[#, S^2<=u v#^2&&#<=v&]&}
]
, {S, 24, SMax, 24}
]//Reap//Last//Last
{x, r, a, b, c, d}=.;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Incorrect Mathematica program removed by Albert Lau, May 25 2016
Missing term 5880 and more terms from Albert Lau, May 25 2016
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STATUS
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approved
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