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電子課本網(wǎng) 第21頁

第21頁

信息發(fā)布者:
解:?$(1)$?由題意,對(duì)于方程?$x^2-6x + 2m - 1 = 0,$?
因?yàn)?$x_{1} = 1,$?根據(jù)根與系數(shù)的關(guān)系?$x_{1}+x_{2}=6,$??$x_{1}·x_{2}=2m - 1。$?
?$ $?則?$1 + x_{2} = 6,$?解得?$x_{2} = 5;$?
?$ 1×x_{2}=2m - 1,$?即?$5 = 2m - 1,$?解得?$m = 3。$?
?$ (2) $?解:存在。
根據(jù)題意,方程?$x^2-6x + 2m - 1 = 0$?中,?$?=(-6)^2-4(2m - 1)\geqslant 0,$?
?$ 36-8m + 4\geqslant 0,$?
?$ 40-8m\geqslant 0,$?
?$ $?解得?$m\leqslant 5。$?
?$ $?假設(shè)存在實(shí)數(shù)?$m,$?滿足?$(x_{1} - 1)(x_{2} - 1)=\frac {6}{m - 5},$?
?$ $?因?yàn)?$x_{1} + x_{2} = 6,$??$x_{1}x_{2} = 2m - 1,$?
?$ $?則?$x_{1}x_{2}-(x_{1} + x_{2})+1=\frac {6}{m - 5},$?
?$ $?即?$2m - 1-6 + 1=\frac {6}{m - 5},$?
?$ 2m-6=\frac {6}{m - 5},$?
?$ (2m - 6)(m - 5)=6,$?
?$ 2\ \mathrm {m^2}-10m-6m + 30 = 6,$?
?$ 2\ \mathrm {m^2}-16m + 24 = 0,$?
?$\mathrm {m^2}-8m + 12 = 0,$?
?$ (m - 2)(m - 6)=0,$?
?$ $?解得?$m_{1} = 2,$??$m_{2} = 6。$?
?$ $?因?yàn)?$m\leqslant 5$?且?$m - 5\neq 0,$?所以?$m = 2。$?
經(jīng)檢驗(yàn),?$m = 2$?是原分式方程的解,且符合題意。
所以假設(shè)成立,即存在實(shí)數(shù)?$m = 2,$?滿足?$(x_{1} - 1)(x_{2} - 1)=\frac {6}{m - 5}。$?
解:?$(1)$?因?yàn)榉匠?$x^2-(2k + 1)x + k^2+2k = 0$?有兩個(gè)實(shí)數(shù)根,
?$ $?所以?$?=b^2-4ac=[-(2k + 1)]^2-4(k^2+2k)$?
?$ =4k^2+4k + 1-4k^2-8k$?
?$ =1 - 4k\geqslant 0,$?
?$ $?解得?$k\leqslant \frac {1}{4}。$?
?$ (2) $?解:不存在。
理由:假設(shè)存在實(shí)數(shù)?$k,$?使得?$x_{1}(x_{2} - x_{1})-x_{2}^2\geqslant 0$?成立。
?$ $?因?yàn)?$x_{1}、$??$x_{2}$?是原方程的兩個(gè)實(shí)數(shù)根,
?$ $?所以?$x_{1} + x_{2} = 2k + 1,$??$x_{1}x_{2} = k^2+2k。$?
?$ $?因?yàn)?$x_{1}(x_{2} - x_{1})-x_{2}^2\geqslant 0,$?即?$x_{1}x_{2}-x_{1}^2-x_{2}^2\geqslant 0,$?
?$ $?所以?$3x_{1}x_{2}-(x_{1} + x_{2})^2\geqslant 0,$?
?$ $?即?$3(k^2+2k)-(2k + 1)^2\geqslant 0,$?
?$ 3k^2+6k-(4k^2+4k + 1)\geqslant 0,$?
?$ 3k^2+6k-4k^2-4k - 1\geqslant 0,$?
?$ -k^2+2k - 1\geqslant 0,$?
?$ k^2-2k + 1\leqslant 0,$?
?$ (k - 1)^2\leqslant 0。$?
?$ $?所以?$k = 1。$?
?$ $?由?$ (1),$?知?$k\leqslant \frac {1}{4},$?
?$ $?所以不存在實(shí)數(shù)?$k,$?使得?$x_{1}(x_{2} - x_{1})-x_{2}^2\geqslant 0$?成立。
解:因?yàn)橐辉畏匠?2x^{2}-4x - 1 = 0$的兩個(gè)實(shí)數(shù)根為$m$、$n,$
所以$2m^{2}-4m - 1 = 0,$$m + n=-\frac{-4}{2}=2,$$mn=-\frac{1}{2},$
所以$2m^{2}-4m = 1,$
所以$3m^{2}-4m + n^{2}=2m^{2}-4m + m^{2}+n^{2}=1+(m + n)^{2}-2mn$
$=1+2^{2}-2\times(-\frac{1}{2})$
$=1 + 4 + 1$
$=6。$