题目

Mr. Peter Johnson, aged twenty-three, battled for half an hour to escape from his trapped car yesterday when it landed upside down in three feet of water．Mr．Johnson took the only escape route—through the boot(行李箱)． Mr．Johnson’s car had finished up in a ditch(沟渠) at Romney Marsin, Kent after skidding on ice and hitting a bank. “Fortunately, the water began to come in only slowly,” Mr Johnson said．“I couldn’t force the doors because they were jammed against the walls of the ditch and dared not open the windows because I knew water would come flooding in. 已知a，b，c为△ABC的三个内角A，B，C的对边，向量m＝（），n＝（cosA,sinA）．若m⊥n，且acosB+bcosA=csinC，则角B＝_______