Tìm số <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mover><mrow><mi>a</mi><mi>b</mi><mi>c</mi></mrow><mo accent="true">―</mo></mover></mrow></math> biết:<br><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mover><mrow><mi>a</mi><mi>b</mi><mi>c</mi></mrow><mo accent="true">―</mo></mover><mo>=</mo><mover><mrow><mi>a</mi><mi>b</mi></mrow><mo accent="true">―</mo></mover><mo>+</mo><mover><mrow><mi>b</mi><mi>c</mi></mrow><mo accent="true">―</mo></mover><mo>+</mo><mover><mrow><mi>c</mi><mi>a</mi></mrow><mo accent="true">―</mo></mover></mrow></math>