# AP Statistics exam

I have a good handle on probability theory, at least I understand the book I 1st course in probablity by Ross.

However, I am having a hard time understanding Statistics, I got a prep book to teach me but it seems to be just logical deductions are given as the reason for why $\frac{\sigma}{\sqrt{n}}$ for the st.dev of a sample for a random sample.

I am asking for any good books online that would give some more mathematical reason why statistics work.

My level: I read and mostly understood the book listed above. I am studying for the AP Statistics exam.