I am reading "Introduction to Electrodynamics" [Griffiths] and in section 9.1.1, there is an explanation for why a stretched string supports wave motion. It begins as follows:
It identifies 2 points, $z$ and $z + \Delta z$ in in the direction that the wave is propagating, and states the net transverse force on the segment between $z$ and $z + \Delta z$ is:
$\Delta F = T (sin\Theta' - sin\Theta)$
so far so good. Then, provided that the distortion of the string is not too great, tan can replace sine because the angles will be small so:
$\Delta F \cong T (tan\Theta' - tan\Theta) = T(\frac{\partial f}{\partial z} - \frac{\partial f}{\partial z})$
where each of the above partial derivatives are taken at points $(z + \Delta z)$ and $z$ respectively.
The next step I am unclear on the reasoning for. It says:
$T(\frac{\partial f}{\partial z} - \frac{\partial f}{\partial z}) \cong T \frac{\partial ^2 f}{\partial z^2} \Delta z$
I suppose I follow this in the sense that both the functions are going to be close to 0, but I feel that there's a more nuanced reasoning for this approximation that I'm not following.
Or perhaps it is that simple and I'm just more accustomed to a more easily geometrically visible reason for an approximation?