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2021年高考数学上海20<-->返回列表
21.(16分)已知x1,x2∈R,若对任意的x2−x1∈S,f(x2)−f(x1)∈S,则有定义:f(x)是在S关联的. (1)判断和证明f(x)=2x−1是否在[0,+∞)关联?是否有[0,1]关联? (2)若f(x)是在{3}关联的,f(x)在x∈[0,3)时,f(x)=x2−2x,求解不等式:2⩽f(x)⩽3. (3)证明:f(x)是{1}关联的,且是在[0,+∞)关联的,当且仅当“f(x)在[1,2]是关联的”. 分析:(1)任取x1−x2∈[0,+∞),证明f(x1)−f(x2)∈[0,+∞),证明f(x)=2x−1在[0,+∞)关联,取x1=1,x2=0,证明f(x)在[0,1]不关联;(2)先得到f(x+3)−f(x)=3,再得到x∈[0,3)和x∈[3,6)的解析式,进而得到答案;(3)先证明f(x)在[1,2]是关联的⇒f(x)是在{1}关联的,且是在[0,+∞)关联的,再证明f(x)在[1,2]是关联的⇒f(x)是在{1}关联的,且是在[0,+∞)关联的. 解:(1)f(x)在[0,+∞)关联,在[0,1]不关联, 任取x1−x2∈[0,+∞),则f(x1)−f(x2)=2(x1−x2)∈[0,+∞),∴在[0,+\infty )关联; 取x_{1}=1,x_{2}=0,则x_{1}-x_{2}=1\in [0,1], \because f(x_{1})-f(x_{2})=2(x_{1}-x_{2})=2\notin [0,1],\therefore f(x)在[0,1]不关联; (2)\because f(x)在\{3\}关联,\therefore对于任意x_{1}-x_{2}=3,都有f(x_{1})-f(x_{2})=3, \therefore对任意x,都有f(x+3)-f(x)=3, 由x\in [0,3)时,f(x)=x^{2}-2x,得f(x)在x\in [0,3)的值域为[-1,3), \therefore f(x)在x\in [3,6)的值域为[2,6), \therefore 2\leqslant f(x)\leqslant 3仅在x\in [0,3)或x\in [3,6)上有解, x\in [0,3)时,f(x)=x^{2}-2x,令2\leqslant x^{2}-2x\leqslant 3,解得\sqrt{3}+1\leqslant x<3, x\in [3,6)时,f(x)=f(x-3)+3=x^{2}-8x+18,令2\leqslant x^{2}-8x+18\leqslant 3,解得3<x\leqslant 5, \therefore不等式2\leqslant f(x)\leqslant 3的解为[\sqrt{3}+1,5], (3)证明:①先证明:f(x)是在\{1\}关联的,且是在[0,+\infty )关联的\Rightarrow f(x)在[1,2]是关联的, \because f(x)是在\{1\}关联的,\therefore当x_{1}-x_{2}=1时,f(x_{1})-f(x_{2})=1,即f(x+1)-f(x)=1, \because f(x)是在[0,+\infty )关联的,\therefore当x_{1}-x_{2}\geqslant 0时,f(x_{1})-f(x_{2})\geqslant 0, 任取x_{1}-x_{2}\in [1,2],即1\leqslant x_{1}-x_{2}\leqslant 2,\therefore x_{1}\geqslant x_{2}+1,x_{1}\leqslant x_{2}+2,\therefore f(x_{2}+1)\leqslant f(x_{1})\leqslant f(x_{2}+2), \therefore f(x_{1})-f(x_{2})\geqslant f(x_{2}+1)-f(x_{2})=1,f(x_{1})-f(x_{2})\leqslant f(x_{2}+2)-f(x_{2})=f(x_{2}+2)-f(x_{2}+1)+f(x_{2}+1)-f(x_{2})=2, \therefore f(x)在[1,2]是关联的; ②再证明:f(x)在[1,2]是关联的\Rightarrow f(x)是在\{1\}关联的,且是在[0,+\infty )关联的, \because f(x)在[1,2]是关联的,\therefore任取x_{1}-x_{2}\in [1,2],都有f(x_{1})-f(x_{2})\in [1,2]成立, 即满足1\leqslant x_{1}-x_{2}\leqslant 2,都有1\leqslant f(x_{1})-f(x_{2})\leqslant 2, 下面用反证法证明f(x+1)-f(x)=1, 若f(x+1)-f(x)>1,则f(x+2)-f(x)=f(x+2)-f(x+1)+f(x+1)-f(x)>2,与f(x)在[1,2]是关联的矛盾, 若f(x+1)-f(x)<1,则f(x+2)-f(x)=f(x+2)-f(x+1)+f(x+1)-f(x)<2,与f(x)在[1,2]是关联的矛盾, \therefore f(x+1)-f(x)=1成立,即f(x)是在\{1\}关联的, 再证明f(x)是在[0,+\infty )关联的, 任取x_{1}-x_{2}\in [n,n+1](n\in N),有1\leqslant x_{1}-(n-1)-x_{2}\leqslant 2, \because f(x)在[1,2]是关联的,\therefore 1\leqslant f[x_{1}-(n-1)]-f(x_{2})\leqslant 2, \because f(x)是在\{1\}关联的,\therefore f(x+1)-f(x)=1,\therefore f(x+k)-f(x)=k, \therefore f[x_{1}-(n-1)]-f(x_{2})=f(x_{1})-(n-1)-f(x_{2})\in [1,2],\therefore n\leqslant f(x_{1})-f(x_{2})\leqslant n+1, \therefore对任意n\in N,f(x)在[n,n+1]是关联的,\therefore f(x)是在[0,+\infty )关联的; 综上所述,f(x)是\{1\}关联的,且是在[0,+\infty )关联的,当且仅当“f(x)在[1,2]是关联的”, 故得证. 点评:该题考查了函数求解析式,解不等式,函数恒成立的知识,对学生逻辑推理能力提出了很高的要求,属于难题.
2021年高考数学上海20<-->返回列表
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