··4分Р令,则,有,···························6分Р, ·····························8分Р即. ·····························10分Р18、填空:; ··························4分Р证明:作,·······················6分Р, ··························6分Р又, ··························8分Р即. ··························10分Р19、⑴, ··························2分Р在中,由正弦定理得:, ························4分Р 即. ··························6分Р⑵, ··························8分Р又由余弦定理得,Р解得, · ·························10分Р. ··························12分Р20、(1)由题意知使用年的维修总费用为Р= 万元··························3分Р 依题得····················6分Р (2) ·························8分Р 当且仅当即时取等号··························10分Р 时取得最小值3 万元Р答:这种汽车使用10年时,它的年平均费用最小,最小值是3万元. ··········12分21、⑴令,则原式Р①当时,;Р②当时,;Р③当时,;Р综上:Р⑵当时,解得,Р当时Р22、⑴令,则Р 数列是为公比为的等比数列.Р⑵,Р,Р,Р.Р⑶设数列的前项和为,Р,Р.Р, 时,,Р时,.
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