Can you do addition/multiplication with Big O notations?

Question

I\'m currently taking an algorithm class, and we\'re covering Big O notations and such. Last time, we talked about how

O (n^2 + 3n + 5) = O(n^2)

Answer 1

Also the following notations hold

O(n^2) + n = O(n^2)

and

O(n^2) + Θ(3n+5) = O(n^2), Θ(n)

Hope it makes sense...