US gross domestic product — the broadest measure of economic activity — grew at an annualized pace of 6.4% in the first three months of the year, adjusted for seasonal swings, the Commerce Department reported Thursday. The government uses the ‘annualized’ rate, which assumed the quarterly growth rate would continue for a full year, because it makes it easier to compare numbers of different periods of time.That was slightly better than economists had predicted, and a faster rate than the