The line `y=mx+b` intersects the x-axis at `((-b)/m,0)` . Let (x,mx+b) be another point on the line, with `x>(-b)/m` .
Drop a perpendicular segment from (x,mx+b) to the x-axis. If the line's x-intercept is labeled A, the point (x,mx+b) labeled B, and the point where the perpendicular segment intersects the x-axis is C then `Delta ABC` is a right triangle. Let the angle at A be `alpha` ; this is the angle between the line and the x-axis.
The geometric definition of the tangent of an acute angle in a right triangle is the ratio of the leg opposite the angle to the leg adjacent to the angle.
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