title: Proving or Refuting Time Complexity
body: <p>I have an exam soon and I wasn't at university for a long time, cause I was at the hospital</p>
  
  <p>Prove or refute the following statements:</p>
  
  <ol>
  <li><p>log(n)= O(
  √
  n)</p></li>
  <li><p>3^(n-1)= O(2^n)</p></li>
  <li><p>f(n) + g(n) = O(f(g(n)))</p></li>
  <li><p>2^(n+1) = O(2^n)</p></li>
  </ol>
  
  <p>Could someone please help me and explain to me ?</p>