360?即?BOC?180??BAD? ··············································································· 14分
n?≌△AD．C ·证法四：同上可证 △ABE································································· 12分
??AEB??ACD．如图，连接CE，??BEC??BOC??OCE ??AEB??AEC??BOC??ACD??ACE ??BOC??AEC??ACE． ······································· 13分
360?即?BOC?180??CAE? ·································· 14分
n?注意：此题还有其它证法，可相应评分．
18.解：l＋2＋…＋61＝1891，2008—1891＝117，由于形如ab的页码被当成ba后，加得的和数将相差9a?b，因为a,b只能在1，2，…，9中取值，a?b≤8，得9a?b≤72，由于117＝72＋45＝63＋54，设弄错的两位数是ab和cd，若9a?b=72，9c?d=45，只有ab＝19，而cd可以取l6，27，38，49；这时ab+cd的最大值是68；若9a?b=63，9c?d=54，则ab可以取18，29，而cd可以取17，28，39，ab+cd的最大值也是68． 备用题： 已知x?
设有理数x,y满足
3432，求x?12x?12x?36x?38的值。（11） 5?221?33?x?y，求xy= （4.5） 4