Big O, what is the complexity of summing a series of n numbers?

Question

I always thought the complexity of:

1 + 2 + 3 + ... + n is O(n), and summing two n by n matrices would be O(n^2).

But today I read from a textbo

Answer 1

The big O notation can be used to determine the growth rate of any function.

In this case, it seems the book is not talking about the time complexity of computing the value, but about the value itself. And n(n+1)/2 is O(n^2).