I am a long snake, terrible to everyone,
I unite everything that has fallen apart.
I am hard to conquer for human, even the smartest ones,
I may help you to find your size.
2 Answers
Are you
An integral.
I am a long snake, terrible to everyone,
The integral sign (∫) looks like an elongated s, or a snake. Integrals can also be quite difficult to calculate, making some people fear them.
I unite everything that has fallen apart.
A derivate measures the sensitivity to change in a point in a function. An integral takes a derivate and unites all the points into the original function again.
I am hard to conquer for human, even the smartest ones,
Calculating integrals is taught as higher education. Even then, learning to use it correctly is often a lot of work.
And it helps to find the size.
Integrals are often used to calculate areas and volumes.
Is the answer
a measuring tape?!
It's almost terrible to everyone since
it either tells you that you are shorter / fatter / slimmer than an average person.
It's hard to conquer
[get yourself slim / fat / tall] to the extent that everybody likes it.
And it helps to find the size.