Generally, the tangential deviation t is not equal to the beam deflection. In cantilever beams, however, the tangent drawn to the elastic curve at the wall is horizontal and coincidence therefore with the neutral axis of the beam. The tangential deviation in this case is equal to the deflection of the beam as shown below.
From the figure above, the deflection at B denoted as δB is equal to the deviation of B from a tangent line through A denoted as tB/A. This is because the tangent line through A lies with the neutral axis of the beam.