江西省乐平市2023-2024学年度八年级下学期期中学业评价数学.考卷答案

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江西省乐平市2023-2024学年度八年级下学期期中学业评价数学.考卷答案试卷答案

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分析求出函数的导数,由函数的零点存在定理可得f′(1)f′(3)<0,进而验证a=4与a=$\frac{16}{9}$时是否符合题意,即可求答案.

解答解:f(x)的导数为f′(x)=$\frac{1}{x}$-$\frac{a}{(x+1)^{2}}$,
当f′(1)f′(3)<0时,函数f(x)在区间(1,3)上只有一个极值点,
即为(1-$\frac{1}{4}$a)($\frac{1}{3}$-$\frac{1}{16}$a)<0,
解得4<a<$\frac{16}{3}$;
当a=4时,f′(x)=$\frac{1}{x}$-$\frac{4}{(x+1)^{2}}$=0,解得x=1∉(1,3),
当a=$\frac{16}{3}$时,f′(x)=$\frac{1}{x}$-$\frac{16}{3(x+1)^{2}}$=0在(1,3)上无实根,
则a的取值范围是4<a<$\frac{16}{3}$,且a∈N,即为a=5.
故选:A.

点评本题考查利用导数研究函数的极值问题,体现了转化的思想方法的运用,考查运算能力,属于中档题.