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 A230361 Integer areas of the tangential triangles corresponding to the integer-sided triangles with integer areas. 0
 23660, 26250, 53235, 94640, 105000, 147875, 212940, 222530, 236250, 378560, 390390, 420000, 479115, 591500, 656250, 788970, 851760, 945000, 1286250, 1330875, 1561560, 1680000, 1916460, 2126250, 2608515, 2625000, 3176250, 3407040, 3513510 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The tangential triangle is the triangle (A', B', C') of side lengths (a', b', c') formed by the lines tangent to the circumcircle of a given triangle (A, B, C) of side lengths (a, b, c) at its vertices. The area of the tangential triangle is given by Area = (1/2)*Area(A,B,C)*|sec A * sec B * sec C| The side lengths of the tangential triangle are: a' = 2*a^3*b*c/|a^4 - (b^2 - c^2)^2| b' = 2*a*b^3*c/|b^4 - (c^2 - a^2)^2| c' = 2*a*b*c^3/|c^4 - (a^2 - b^2)^2| In the general case, these sides lengths are rational numbers (see the examples in the table below). But it is possible to find integer sides, for example the triangle (210, 210, 252) generates a tangential triangle (625, 625, 350). It is possible to find the same tangential triangle with two distinct triangles of side lengths (a1,b1,c1) and (a2,b2,c2); for example, the triangles (105, 105, 175) and (140, 140, 224) generate the same tangential triangle (625/2, 625/2, 175). The following table gives the first values (S', S, a, b, c, a', b', c') where S' is the area of the tangential triangle (A', B', C'), S is the area of the initial triangle (A, B, C), a, b, c the integer sides of the triangle (A, B, C) and a', b', c' are the integer sides of the tangential triangle (A', B', C'). ************************************************************** S' * S * a * b * c * a' * b' * c' ************************************************************** 23660 * 10584 * 84 * 273 * 315 * 182 * 2197/6 * 1625/6 26250 * 5292 * 105 * 105 * 126 * 625/2 * 625/2 * 175 26250 * 9408 * 140 * 140 * 224 * 625/2 * 625/2 * 175 26250 * 24192 * 168 * 360 * 480 * 175 * 625/2 * 625/2 53235 * 8064 * 104 * 160 * 168 * 2197/4 * 1625/4 * 273 94640 * 42336 * 168 * 546 * 630 * 364 * 2197/3 * 1625/3 105000 * 21168 * 210 * 210 * 252 * 625 * 625 * 350 105000 * 37632 * 280 * 280 * 448 * 625 * 625 * 350 147875 * 40320 * 200 * 416 * 504 * 8125/12 * 10985/12 * 455 212940 * 32256 * 208 * 320 * 336 * 2197/2 * 1625/2 * 546 ......................................... REFERENCES Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, 1929. Kimberling, C. Triangle Centers and Central Triangles. Congr. Numer. 129, 1-295, 1998. LINKS Table of n, a(n) for n=1..29. C. Kimberling, Central Points and Central Lines in the Plane of a Triangle, Math. Mag. 67, 163-187, 1994. Eric W. Weisstein, MathWorld: Tangential Triangle EXAMPLE 26250 is in the sequence because the triangle of integer sides (a, b, c)= (105, 105, 126) generates the tangential triangle (a', b', c') = (625/2, 625/2, 175) where: a' = 2*a^3*b*c/|a^4 - (b^2 - c^2)^2| = 625/2 b' = 2*a*b^3*c/|b^4 - (c^2 - a^2)^2| = 625/2 c' = 2*a*b*c^3/|c^4 - (a^2 - b^2)^2| = 175 The area of this triangle is given by two possible ways: First way: Heron's formula gives Area = sqrt(s'*(s'-a')*(s'-b')*(s'-c')) = sqrt(400*(400-625/2)*(400-625/2)*(400-175)) = 26250 with the semiperimeter s' = (625/2 + 625/2 + 175)/2 = 400. Second way: Area of the triangle (a, b, c) = sqrt(sqrt(s*(s-a)*(s-b)*(s-c))=sqrt(168*(168-105)*(168-105)*(168-126))=5292 with the semiperimeter s = (105 + 1052 + 126)/2 = 168. Then, we use the formula Area = (1/2)* Area(A,B,C) * |sec A * sec B * sec C| = 2646*5/3*5/3*25/7 = 26250 where: sec A = 1/cos A = 2*b*c/(b^2+c^2-a^2)= 5/3; sec B = 1/cos B = 2*a*c/(c^2+a^2-b^2)= 5/3; sec C = 1/cos C = 2*a*b/(a^2+b^2-c^2)= 25/7. MATHEMATICA nn=1500; lst={}; Do[s=(a+b+c)/2; If[IntegerQ[s], area2=s (s-a) (s-b) (s-c); aa=Abs[((b^2+c^2-a^2)*(c^2+a^2-b^2)*(a^2+b^2-c^2))]; If[0 < area2 && aa>0&& IntegerQ[Sqrt[area2]* (4*a^2*b^2*c^2)/aa], AppendTo[lst, Sqrt[area2]* (4*a^2*b^2*c^2)/aa]]], {a, nn}, {b, a}, {c, b}]; Union[lst] CROSSREFS Cf. A188158. Sequence in context: A237508 A177827 A304652 * A069334 A118061 A251855 Adjacent sequences: A230358 A230359 A230360 * A230362 A230363 A230364 KEYWORD nonn AUTHOR Michel Lagneau, Oct 16 2013 STATUS approved

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