title: How do we calculate the new values for theta(weights?) in the output layer after backpropagation?
body: <p>I'm currently trying to catch up with Andrew Ng's machine learning course on Coursera and I'm having a little bit of trouble...</p>
  
  <p>In his backpropagation video he explains that we need to calculate big_delta</p>
  
  <p><code>big_delta = big_delta + delta(l+1) * a(l)^t</code> for all layers l</p>
  
  <p>However, how do I calculate big_delta for the output layer when I don't have delta(l+1)?</p>
  
  <p>The video where he explains this is here: <a href="https://www.youtube.com/watch?v=wmfpS5fKFeY" rel="nofollow">https://www.youtube.com/watch?v=wmfpS5fKFeY</a></p>
  
  <p>I'm lost. </p>