答案一、选择题DDCBCBCACCDA二、填空题6-13)414)n21-2515)1816)三、解答题17、(1)由正弦定理:3,53sinsincBCbc得5b---------------------------------------------------------5分(2)由余弦定理:212cos222bcacbA,32A0A),,(-------------------------------5分18、(1)202nan------------------------------------------------4分(3)由(1)知:0202nan得10n所以当时或109nn------------------------2分90minns-----------------------------------------------4分19、(1)当1,2yx时----------------------------------------2分5maxu---------------------------------------------1分当3,2yx时----------------------------------------2分9minu-----------------------------------------1分(2)223111)2(222d-----------------------------3分29minv--------------------------------------------------3分20、(1)3C--------------------------------------------------