I need help with the following problem :

Question

f(n) = n^2 + 4n is O(n^2)

Answer 1

See here:

http://en.wikipedia.org/wiki/Big_O_notation#Formal_definition

You need to find an N and an M > 0 such that

n^2 + 4n < M n^2

for all n > N

This looks like a trivial problem to me...