Let me explain. It is generally written as i= 1, which means to sum from 1.

It's usually n, which means sum to n.

It will be more helpful for your understanding to give a concrete example.

For example, ∑(ai) (subscript is i= 1 with n on it), then the result is:

a 1+a2+a3+…+an

Take another example ∑ (a I) (subscript is i=5, and above it is 10), then the result is:

a^5+a^6+a^7+a^8+a^9+a^ 10.

Now you should understand, right? If you don't understand anything, please continue to ask me questions through SMS. I am a teacher, and I will have the patience and experience to answer any questions you ask. Thank you.