In a triangle \( ABC \), the coordinates of the vertex \( A \) are \( (1, 2) \). Equations of the median through \( B \) and \( C \) are respectively \( x + y = 5 \) and \( x = 4 \). Then the equation of side \( AB \) is:
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To find the equation of a line between two points, first find the slope using \( m = \frac{y_2 - y_1}{x_2 - x_1} \), then use the point-slope form to get the equation of the line.