derivable vs piece of cake

derivable

adj
  • able to be derived, deducible 

piece of cake

noun
  • Used other than figuratively or idiomatically: see piece, cake. 

  • A job, task or other activity that is pleasant – or, by extension, easy or simple. 

How often have the words derivable and piece of cake occurred in a corpus of books? (source: Google Ngram Viewer )