Infinitely small does not exist
The Pythagorean theorem also works for infinitely small numbers, because
Explanation
If you want to get from A to B, you can walk 1 km east and then 1 km north. You will then cover 2 km.
B 1 km A 1 km
If you first walk to the east, then to the north, and so on, the distance travelled remains 2 km.
B A
Even if you make the sections shorter and shorter, the distance travelled remains 2 km.
B A
No matter how small you make the sections, the distance remains the same.
B A
Only when you take the diagonal does the distance become smaller. You then don't walk first to the east and then to the north, but directly to the north-east. You cover in that way only 1.414213562… km.
B A
You might expect that if the sections become infinitely small, then the distance traveled must still remain 2 km. But that is a fallacy. Also in an infinitesimal square, the ratio of the infinitesimal side to the infinitesimal diagonal is 1 : √2.
Infinitely small does not exist and infinity is not a number. You may calculate with it, but of course you must do it correctly.