Hard-charging former businessman Lee Myung-bak became South Korea's new president on Monday with a mandate to boost the economy and take a tougher line on nuclear-

　　armed North Korea.

　　The conservative, pro-US Lee, nicknamed "The Bulldozer" for the can-do image he honed as a construction company CEO and later as mayor of Seoul, took the oath of office amid a booming artillery salute at the National Assemblyon Monday.