LinkedIn is buying Lynda.com for about $1.5 billion in a cash-and-stock deal, adding the online learning and professional development company to its professional networking offerings.

Mountain View, California-based LinkedIn said Thursday that it will pay for the Carpinteria, California-based company with a combination of 52 percent cash and 48 percent stock. The deal is expected to close during the second quarter. It is by far LinkedIn's largest acquisition so far, and one that should help the 12-year-old company expand its audience and the market it serves.

"We believe this deal makes a lot of sense for the leading professional network, since it would empower employees/subscribers to develop or further refine their skills (instead of simply reporting them on a profile page)," wrote Cantor Fitzgerald analyst Youssef Squali in a note to investors. LinkedIn's key focus is online professional networking. Lynda.com is a subscription service that allows members to access courses taught by experts, in multiple languages. LinkedIn estimates the professional certification and employee education market to be as much as $30 billion. Squali thinks the two companies are well-aligned in their mission "building out the economic graph." That is, if Facebook has mapped people's social connections online, LinkedIn can do the same for professional relationships whether that's between people or businesses. "Given LinkedIn's interest in learning/development, and both companies' belief that the skills gap is one of the greatest challenges of our time, we believe the combination of the two makes a lot of sense," Squali wrote.