本文为整理,非原创,解法均收集自网络.来自兰琦博客 (2008年·江西·理)已知函数f(x)=11+x−−−−−√+11+a−−−−√+axax+8−−−−−−√f(x)=11+x+11+a+axax+8. (1)当a=8a=8,求f(x)f(x)的单调区间; (2)对任意正数aa,证明:1<f(x)<21<f(x)<2. 令b=xb=x,c=8axc=8ax,则第(2)问等价于: 若a,b,c>0a,b,c>0,abc=8abc=8,求证: 1<11+a−−−−√+11+b−−−−√+11+c−−−−√<2.1<11+a+11+b+11+c<2. 该不等式与2004年西部奥林匹克最后一题: 设a,b,c>0a,b,c>0,求证: 1<aa2+b2−−−−−−√+bb2+c2−−−−−−√+cc2+a2−−−−−−√⩽32–√2.1<aa2+b2+bb2+c2+cc2+a2⩽322. 类似,且证明比这道西部奥林匹克题还难.而这道西部奥林匹克题当年参赛选手无一人完全证出.另外,CMO2003第三题: 给定正整数nn,求最小的正数λλ,使得对于任何θi∈(0,π2)θi∈(0,π2)(i=1,2,⋯,ni=1,2,⋯,n),只要tanθ1⋅tanθ2⋯tanθn=2n2tanθ1⋅tanθ2⋯tanθn=2n2,就有cosθ1+cosθ2+⋯+cosθncosθ1+cosθ2+⋯+cosθn不大于λλ. 答案是:当n⩾3n⩾3时,λ=n−1λ=n−1;当n=3n=3时,令 a=tan2θ1,b=tan2θ2,c=tan2θ3,a=tan2θ1,b=tan2θ2,c=tan2θ3, 即得江西压轴题右边的不等式.命题人陶平生教授的证明: 对任意给定的a>0a>0,x>0x>0,因为 f(x)=11+x−−−−−√+11+a−−−−√+11+8ax−−−−−−√,f(x)=11+x+11+a+11+8ax, 若令b=8axb=8ax,则abx=8abx=8,且f(x)=11+x−−−−−√+11+a−−−−√+11+b−−−−√.f(x)=11+x+11+a+11+b. 先证f(x)>1f(x)>1. 因为 11+x−−−−−√>11+x,11+x>11+x, 又由2+a+b+x⩾42abx−−−−√4=8,2+a+b+x⩾42abx4=8, 得a+b+x⩾6.a+b+x⩾6. 所以f(x)=11+x−−−−−√+11+a−−−−√+11+b−−−−√>11+x+11+a+11+b=3+2(a+b+x)+(ab+bx+xa)(1+x)(1+a)(1+b)⩾9+(a+b+x)+(ab+bx+xa)(1+x)(1+a)(1+b)=1+(a+b+x)+(ab+bx+xa)+abx(1+x)(1+a)(1+b)=1.f(x)=11+x+11+a+11+b>11+x+11+a+11+b=3+2(a+b+x)+(ab+bx+xa)(1+x)(1+a)(1+b)⩾9+(a+b+x)+(ab+bx+xa)(1+x)(1+a)(1+b)=1+(a+b+x)+(ab+bx+xa)+abx(1+x)(1+a)(1+b)=1. 再证明f(x)<2f(x)<2. 由x,a,bx,a,b的对称性,不妨设x⩾a⩾bx⩾a⩾b,则0<b⩽20<b⩽2. 情形1:当a+b⩾7a+b⩾7时,此时x⩾a⩾5x⩾a⩾5. 因此 11+b−−−−√<1,11+x−−−−−√+11+a−−−−√⩽21+5−−−−√<1,11+b<1,11+x+11+a⩽21+5<1, 此时f(x)=11+x−−−−−√+11+a−−−−√+11+b−−−−√<2.f(x)=11+x+11+a+11+b<2. 情形2:当a+b<7a+b<7时,x=8abx=8ab,11+x−−−−−√=abab+8−−−−−−√11+x=abab+8. 因为 11+b<1−b1+b+b24(1+b)2=[1−b2(1+b)]2,11+b<1−b1+b+b24(1+b)2=[1−b2(1+b)]2, 所以11+b−−−−√<1−b2(1+b).11+b<1−b2(1+b). 同理得11+a−−−−√<1−a2(1+a),11+a<1−a2(1+a), 于是f(x)<2−12(a1+a+b1+b−2abab+8−−−−−−√).f(x)<2−12(a1+a+b1+b−2abab+8). 而 a1+a+b1+b⩾2ab(1+a)(1+b)−−−−−−−−−−−−√=2ab1+a+b+ab−−−−−−−−−−−−√>2abab+8−−−−−−√,a1+a+b1+b⩾2ab(1+a)(1+b)=2ab1+a+b+ab>2abab+8, 因此不等式得证.综上所述,原不等式得证. 张景中院士的作法: 原问题即三个正数a,b,ca,b,c在abc=8abc=8的条件下求 F(a,b,c)=11+a−−−−√+11+b−−−−√+11+c−−−−√F(a,b,c)=11+a+11+b+11+c 的取值范围.不妨设a⩽b⩽ca⩽b⩽c,记t=at=a,k=abk=ab,c=8kc=8k,把F(a,b,c)F(a,b,c)看成是关于tt的函数 f(t)=11+t−−−−√+11+kt−−−−−√+11+8k−−−−−√,f(t)=11+t+11+kt+11+8k, 注意到变量和参数的范围是0<t⩽k−−√⩽2,0<t⩽k⩽2, 计算导数f′(t)=12(−(1+t)−32+kt2(1+kt)−32)=[k2(1+t)3−t(t+k)3]⋅Q(t,k),f′(t)=12(−(1+t)−32+kt2(1+kt)−32)=[k2(1+t)3−t(t+k)3]⋅Q(t,k), 这里Q(t,k)Q(t,k)是某个正值代数式.于是可以根据g(t)=k2(1+t)3−t(t+k)3g(t)=k2(1+t)3−t(t+k)3 的正负来判断f(x)f(x)的单调性.注意到g(k−−√)=0g(k)=0,因式分解为 g(t)=(k−t2)[t2−k(k−3)t+k],g(t)=(k−t2)[t2−k(k−3)t+k], 由于第二个因式的Δ=k2(k−3)2−4k,Δ=k2(k−3)2−4k, 当k<4k<4时有Δ<0Δ<0,于是有f(x)f(x)在(0,k−−√)(0,k)上单调递增,从而f(t)f(t)在t=k−−√t=k处最大.从而可得 f(t)>1f(t)>1 以及f(1+k−−√)=21+k−−√−−−−−−√+11+8k−−−−−√<2.f(1+k)=21+k+11+8k<2. 原命题得证.不妨设a⩽b⩽ca⩽b⩽c,令ab−−√=λab=λ,由abc=8abc=8有0<λ⩽20<λ⩽2. 设 A=11+a−−−−√+11+b−−−−√,A=11+a+11+b, 再令u=1+a+b+λ2−−−−−−−−−−−√,u=1+a+b+λ2, 则u⩾1+λ,u⩾1+λ, 所以A2=(1−λ2)+2u+1,A2=(1−λ2)+2u+1, 又令t=1u,0<t⩽1λ+1t=1u,0<t⩽1λ+1,则A2=F(t)=(1−λ2)t2+2t+1,A2=F(t)=(1−λ2)t2+2t+1, 求导有F′(t)=2(1−λ2)t+2,F′(t)=2(1−λ2)t+2, 由λ⩽2λ⩽2有F′(1λ+1)=2−λ⩾0,F′(1λ+1)=2−λ⩾0, 又F′(0)=1>0F′(0)=1>0,因此,对0<t⩽1λ+10<t⩽1λ+1,有F′(t)⩾0F′(t)⩾0,所以F(t)F(t)在(0,1λ+1](0,1λ+1]是增函数,则F(0)<F(t)⩽F(1λ+1),F(0)<F(t)⩽F(1λ+1), 即1<A⩽2λ+1−−−−−√,1<A⩽2λ+1, 即当a,b>0a,b>0且ab−−√⩽2ab⩽2时,有1<11+a−−−−√+11+b−−−−√⩽21+ab−−√−−−−−−−√.1<11+a+11+b⩽21+ab. 应用该不等式立得欲证明不等式的左边. 要证明不等式的右边,还需证明 21+ab−−√−−−−−−−√+11+8ab−−−−−−√<2,21+ab+11+8ab<2, 令1+ab−−√−−−−−−−√=v,1<v⩽3–√1+ab=v,1<v⩽3,则ab=(v2−1)3,ab=(v2−1)3, 此不等式等价于2v+v2−1(v2−1)2+8−−−−−−−−−−√<2,2v+v2−1(v2−1)2+8<2, 用分析法容易证得.综上,原不等式得证. 令 u=11+a−−−−√,v=11+b−−−−√,w=11+c−−−−√,u=11+a,v=11+b,w=11+c, 则a=1u2−1,b=1v2−1,c=1w2−1,a=1u2−1,b=1v2−1,c=1w2−1, 且(1u2−1)(1v2−1)(1w2−1)=8.(1u2−1)(1v2−1)(1w2−1)=8. 假设存在a,b,c>0a,b,c>0且abc=8abc=8,但左边不等式不成立,即存在u,v,wu,v,w满足上式,但 u+v+w⩽1.u+v+w⩽1. 注意到0<u,v,w<10<u,v,w<1,有 1+u>1−u⩾v+w⩾2vw−−−√>0,1+u>1−u⩾v+w⩾2vw>0, 所以1u2−1=(1−u)(1+u)u2>(1−u)2u2⩾4vwu2,1u2−1=(1−u)(1+u)u2>(1−u)2u2⩾4vwu2, 同理1v2−1>4wuv2,1w2−1>4uvw2,1v2−1>4wuv2,1w2−1>4uvw2, 从而可得(1u2−1)(1v2−1)(1w2−1)>61,(1u2−1)(1v2−1)(1w2−1)>61, 矛盾,因此左边不等式得证.假设存在a,b,c>0a,b,c>0且abc=8abc=8,但右边不等式不成立,即存在u,v,wu,v,w满足上式,但 u+v+w⩾2.u+v+w⩾2. 注意到0<u,v,w<10<u,v,w<1,有 0<1+u<2,0<1+u<2, 进而0<1−u⩽v+w−1=vw−(1−v)(1−w)<vw,0<1−u⩽v+w−1=vw−(1−v)(1−w)<vw, 从而0<1u2−1(1−u)(1+u)u2<2vwu2,0<1u2−1(1−u)(1+u)u2<2vwu2, 同理0<1v2−1<2wuv2,0<1w2−1<2uvw2,0<1v2−1<2wuv2,0<1w2−1<2uvw2, 从而可得(1u2−1)(1v2−1)(1w2−1)<8,(1u2−1)(1v2−1)(1w2−1)<8, 矛盾,因此右边不等式得证.综上,原不等式得证. |
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