President Reagan and Canadian Prime Minister Pierre Trudeau yesterday smoothed over important differences on bilateral as well as world economic issues, promising a cooperative effort to make a success of the seven-nation summit that begins July 19 in Ottawa.

After a morning meeting and working luncheon at the White House lasting two hours overall, Reagan and Trudeau appeared on the North Portico to stress the need for paternership in solving worldwide problems of inflation and unemployment. But only Trudeau, who will be chairman of the summit, hinted at the "areas of possible disagreement" on global problems that could come up in Ottawa.

The purpose of Trudeau's visit was to touch base hear as part of a presummit series of meetings he has held in West Germany, Britain, France and elsewhere. After meeting with Reagan, Trudeau conferred with Gaston Thorn, president of the Common Market, who also will attend the summit.

Trudeau apparently dealt only gently with the issue that is uppermost in the minds of European leaders: the impact of high U.S. interest rates on the economies of Europe.

Canadian officials had said yesterday morning, before the visit, that European leaders believe they are "at an economic crossroads" because of the high interest rates and will insist at the summit that the United States show more sensitivity to the international impact of its actions.

But American and Canadian sources said Trudeau decided that he did not need to belabor this point. "The message has gotten across," a senior American official said.

What also came across, with striking force, is the sharp split between the U.S. approach to increased aid to the less-developed nations and the attitude of Trudeau, supported by French President Francois Mitterand and most of the other summit partners.

A senior American official said Reagan had told Trudeau that the best way to help the poor countries is to demonstrate that they can increase their economic growth largely by their own efforts.

According to the official, Reagan observed that the United States and Canada once were underdeveloped countries, "and through hard work and the role of the private-enterprise system" rapidly became world powers.

The official added that the United States is "not accustomed to the concept of "global negotiations" for Third World aid that the Canadians and the Europeans are eager to put on the Ottawa agenda. In fact, the official said. there is no clear meaning to the term. "For some global negotiations may be more of a state of mind than a reality," he said.

Canadian officials took sharp issue with this, observing that "global negotiation," recently endorsed by the European Community, "is a well-known concept in which all nations participate in a discussion of a number of themes of interest to the Third World, so they don't have to take them up separately."