NEW YORK — Dollar Tree is buying rival discount store Family Dollar in a cash-and-stock deal valued at about $8.5 billion.

Stockholders of Family Dollar will receive $59.60 in cash and the equivalent of $14.90 in shares of Dollar Tree for each share they own. The companies put the value of the transaction at $74.50 per share.

The boards of both companies have unanimously approved the deal, which is expected to close by early next year.

DETROIT — The U.S. government’s highway safety agency has opened a formal investigation into air bag failures in some Chevrolet Impalas.The National Highway Traffic Safety Administration said Friday it began the probe after receiving a petition from Donald Friedman of Xprts LLC, a Santa Barbara, Calif., company that examines crashes.Friedman examined a 2011 car crash in Texas that severely injured an elderly man. His wife was driving their 2008 Impala when it was hit by an SUV and forced into a divider and a fixed barrier in front of the car. The passenger air bags didn’t deploy and the man suffered permanent brain injuries, according to a lawsuit filed by the couple against GM. He died about 10 months later.The petition asked NHTSA to investigate and recall the cars that use the same computer to sense a passenger’s weight. It says GM used the same system from 2004 through 2010.