title: Expressions about algorithm complexity
body: <p>I would like to know ways to prove it. Because I have no idea:</p>
  <p>n + log n  is Θ(n);</p>
  <p>100n is Ω(n2) ;
  f(n) is O(f(n/2));</p>
  <p>For the two functions g(n) and f(n).f(n)=Ω(g(n)) If and only if f(n)=O(g(n)).</p>