20.解:(1)因为f(x)=,x∈(0,+∞),……:1分所以f(x)在(0,1)上单调递减,在(1,+∞)上单调递增,……3分故f(x)==f(1)=e,f(x)无最大值。…………:4分(2)因为k(x)=f(x)-g(x)=-(ax-alm2x),所以-12-(-1)-(=)(-)又x∈(0,1)6分要使得h(x)在(0,1)内有且仅有一个极值点,则h(x)=0在(0,1)内存在唯一变号实根,即方程-a=0在(0,1)内存在唯一实数解,即y=与y=a在x∈(0,1)内有唯一交点,9分由(1)知:当x∈(0,1),f(x)>f(1)当x0°时,f(x)→+∞……………………10分所以当a∈(e,+∞)时,工与y=在x∈(0,)内有唯一交11分综上所述:实数a的取值范围为(e,+∞),……12分21.解:(1)因为MN⊥FF2

书面表达One possible versionInternational studentsTo enrich our school life and build our body, a hiking with the theme of"Experience the Hutuo River Cul-re"will be held this weekend.As scheduled, any student willing to participate in the activity should gather at the school gate at 8: 00 amon Saturday. We'll walk along the bank of the Hutuo River, going all the way north. At noon, we'll reach abeautiful town, where some experts will tell us history and culture about the river. Finally, we ' ll return by thesame routeMatters needing attention are as follows: uniforms, comfortable shoes, enough water etc.wish you a good journey.

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