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语法填空SpaceX launches two NASA astronauts to spacefor the first time in historic US missionSpaceX has begun new chapter in the history of United States space flight.Elon Musk's private space company on Saturday launched NASA (astronaut) Bob Behnken and Doug Hurley into orbit, successfully (begin) SpaceX's first crewed mission.The company's Falcon 9 rocket and Crew Dragon spacecraft took off from NASA's Kennedy Space Center in Florida 3∶22 p. m.ET.The capsule is the first (private) designed and built spacecraft (carry) astronauts to space and is bound for the International Space Statio定义：我们把满足an+an-1=k（n≥2，k是常数）的数列叫做等和数列，常数k叫做数列的公和．若等和数列{an}的首项为1，公和为3，则该数列前2010项的和S2010=    ．