Formula correction, D.4 summary/flashcards, and more new problems

The formula for the CV(k) error incorrectly showed a 1/n in the front of the sum, where it should have been a 1/k, as in the lesson.

This has been changed on the D.4 summary sheet, and on the flashcard for this formula. The flashcard will automatically update in the flashcard app, and the printed flashcards have been updated, although you may just want to write in this change rather than to try to reprint a single card.

Additionally, more problems have been added this week to lesson D.5.1, D.6.1 and D.6.2.