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 A343063 Primitive triples (a, b, c) for integer-sided triangles whose angle B = 2*C. 5
 5, 6, 4, 7, 12, 9, 9, 20, 16, 11, 30, 25, 13, 42, 36, 15, 56, 49, 16, 15, 9, 17, 72, 64, 19, 90, 81, 21, 110, 100, 23, 132, 121, 24, 35, 25, 25, 156, 144, 27, 182, 169, 29, 210, 196, 31, 240, 225, 32, 63, 49, 33, 28, 16, 33, 272, 256, 35, 306, 289, 37, 342, 324, 39, 40, 25, 39, 380, 361, 40, 99, 81, 41, 420, 400, 43, 462, 441 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence is inspired by the problem of French Baccalauréat Mathématiques at Lyon in 1937 (see link). The triples (a, b, c) are displayed in increasing order of side a, and if sides a coincide then in increasing order of the side b. If in triangle ABC, B = 2*C, then the corresponding metric relation between sides is a*c + c^2 = c * (a + c) = b^2. This metric relation is equivalent to a = m^2 - k^2, b = m * k, c = k^2, gcd(m,k) = 1 and k < m < 2k; hence every c is a square number. When A <> 45° and A <> 72°, table below shows there exist these 3 possible inequalities: c < b < a, c < a < b, a < c < b.    ------------------------------------------------------------------------    | A | 180 |   decr.  |   72    |   decr.   |   45    |   decr.   |  0  |    ------------------------------------------------------------------------    | B |  0  |   incr.  |   72    |   incr.   |   90    |   incr.   | 120 |    ------------------------------------------------------------------------    | C |  0  |   incr.  |   36    |   incr.   |   45    |   incr.   |  60 |    ------------------------------------------------------------------------    | < | No | c < b < a | c < b=a | c < a < b | c=a < b | a < c < b |  No |    ------------------------------------------------------------------------ where 'No' means there is no such corresponding triangle. If (A,B,C) = (72,72,36) then a = b = c * (1+sqrt(5))/2 and isosceles ABC is not an integer-sided triangle. If (A,B,C) = (45,90,45) then ABC is isosceles rectangle in B, so a = c with b = a*sqrt(2) and ABC is not an integer-sided triangle. REFERENCES V. Lespinard & R. Pernet, Trigonométrie, Classe de Mathématiques élémentaires, programme 1962, problème B-336 p. 178, André Desvigne. LINKS EXAMPLE The smallest such triangle is (5, 6, 4), of type c < a < b with 4*(5+4) = 6^2. The 2nd triple is (7, 12, 9) of type a < c < b with 9*(7+9) = 16^2. The 7th triple (16, 15, 9) is the first of type c < b < a with 9*(16+9) = 15^2. The table begins:    5,  6,  4;    7, 12,  9;    9, 20, 16;   11, 30, 25,   13, 42, 36;   15, 56, 49;   16, 15,  9;   17, 72, 64;   ... MAPLE for a from 2 to 60 do for c from 3 to floor(a^2/2) do d := c*(a+c); if igcd(a, sqrt(d), c)=1 and issqr(d) and abs(a-c)

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Last modified June 17 16:32 EDT 2021. Contains 345085 sequences. (Running on oeis4.)