若x，y∈R，且 ，则z=x-2y的最大值等于

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='13603' and id < 1103968 and id<>'1103968' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='13603' and id > 1103968 and id<>'1103968' order by id asc limit 5