What is the essence of "infinitesimal method" in senior high school? In advanced mathematics? what's up "difference"

Infinitesimal method? Medium? what's up △x？ Is it expressed in differential terms? Advanced (short for deluxe)

In the infinitesimal method? Speed? V=△x/△t expressed by differential? Is it dx/dt?

Infinitesimal method? Medium? Generally based on? When infinity approaches zero? "Approximate"? Coming? Did you find it? Every infinitesimal element? The relationship between. And then what? Where did the "differential" come from? To find every one? Differential? The relationship between.

Take this question as an example:?

(1) Use the infinitesimal method:? The speed of the rope v 1=△S/△t? The speed of the ship? V=△x/△t is determined by the geometric relationship, when? When both △s and △x are small, are they approximately satisfied? △s=△x？ Cosα so v 1=vcosα？

(2) use? Differential? Let's ask: v 1 = DS/DTV = DX/DTS- rope length? X-distance from the ship's shore? Settings? How high is the shore? H the angle between the rope and the water surface is α? then what S=h/sinαx=h/tanα

So what? v 1=ds/dt=(-h？ cosα/sin？ α? )dα/dtv=dx/dt=[-？ H (sin? α+cos？ α)/sin？ α]dα/dt=？ (-h/sin？ α? )dα/dt

So what? v 1/v=？ What is Cosα? v 1 = VCOα

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