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Example of a subtangent

The subtangent is a term from analytical geometry . We consider a point on a differentiable curve (red) and form the tangent (green). The projection of the tangent onto the abscissa is then called the subtangent (yellow in the picture).

If a curve is differentiable at one point , the subtangent is the distance between the point on the abscissa and the zero point of the tangent.

The zero and thus the subtangent can be obtained via the equation of the tangent

The amount of the subtangent of the function is constant 1 for all tangents, since:

As an analogy to the normal there are the subnormal .