## Subtangent and Subnormal

Given a curve, the subtangent at a point
$(x,y)$
on the curve is the interval on the
$x$
axis between
$x$
and the intercept
$x_T$
of the tangent with the
$x$
axis, and the subnormal is the interval on the
$x$
axis between
$x$
and the intercept of the normal with the axis.