Chicago--The Tribune Co. and the Times Mirror Co., Los Angeles, announced Monday that they have agreed to merge in a cash and stock deal valued at $8 billion. The new company will own newspapers in the three largest markets in the United States: New York (Newsday
), Los Angeles (The Los Angeles Times
), and Chicago (Chicago Tribune
). The merger comes in an era when business-to-business marketers are increasingly using business sections in local newspapers as part of national media buys.