the part of the x-axis cut off between the ordinate of a given point of a curve and the tangent at that point.
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British Dictionary definitions for subtangent
geometry a segment of the x -axis lying between the x -coordinate of the point at which a tangent is drawn to a curve and the intercept of the tangent with the axis
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012