Cho a + b + c = 0. Tính A = \(\frac{ab}{a^2 + b^2 - c^2}\) + \(\frac{bc}{b^2 + c^2 - a^2}\) + \(\frac{ca}{c^2 + a^2 - b^2}\)

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Cho a + b + c = 0. Tính

A = \(\frac{ab}{a^2 + b^2 - c^2}\) + \(\frac{bc}{b^2 + c^2 - a^2}\) + \(\frac{ca}{c^2 + a^2 - b^2}\)