The triangle ABC is known, and its area is s = 1 / 2a2-1 / 2 (B-C) 2, Sina

S = (1 / 2) a ^ 2 - (1 / 2) (B-C) ^ 2 = (1 / 2) bcsina, ｜ Sina = [a ^ 2 - (B-C) ^ 2] / BC = [2BC - (b ^ 2 + C ^ 2-A ^ 2)] / BC = 2-2cosa, ｜ Sina + 2cosa = 2, ｜ (√ 5) sin (a + T) = 2, where Sint = 2 / √ 5, cost = 1 / √ 5

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