书面表达One possible versionDear JackI am glad to hear that you will come to our school as an exchange student next year. Since you want toknow about my school, i am writing to introduce some details to you.My school lies in the southeast of our city with convenient transportation, so it will be easy for you to comefrom the airport. The school, where various flowers and fruit trees can be found, provides a pleasantenvironment for students to study in. My school has three grades and almost 1, 000 students altogether andnearly two-thirds of graduates are admitted to key universities every year. Above all, they are friendly andhelpful. Therefore, if you have any difficulty adjusting to your future life, everyone, including me, will give youa handLooking forward to your coming.YoursLi hi

21.【解析】(1)由题意,f(0)=10,解得a=2,(2分)1+1所以f(x)=/--24-11+44+1所以f(-x)=f(x),4-+11+4所以函数f(x)为奇函数,符合题意.(4分)(2)由(1)f(x)=/-2,易知∫(x)在R上单调递增.(5分)1+42因为f(k>0Care4+以∫(kf()=f(以k4+21>即k(-2)即(2k+1)2+4.22-k>0在R上恒成立,(7分)令t=2>0,则(2k+1)2+4-k>0,对1>0时恒成立,①当2k+1=0,即k=-时,4+->0,对t>0时恒成立;(8分)②当2k+1≠0,即k≠一时,2k+1>02k+1>0由题意,得或

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