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Chị gì gì ơi những bài toán khó như vậy chị nên đăng trên H.VN
Ở đó học sinh lớp 9,10,8,7 sẽ giúp cho
Ta có \(\Delta'=\left(m-1\right)^2-2m+5\ge0\)
=> \(m^2-4m+6\ge0\)luôn đúng
Theo vi-et ta có \(\hept{\begin{cases}x_1+x_2=2\left(m-1\right)\\x_1x_2=2m-5\end{cases}}\)
Khi đó
\(P=\left(\frac{x_1}{x_2}+\frac{x_2}{x_1}\right)^2-2\)
\(=\left(\frac{\left(x_1+x_2\right)^2-2x_1x_2}{x_1x_2}\right)^2-2\)
\(=\left(\frac{4\left(m-1\right)^2}{2m-5}-2\right)^2-2\)
\(=\left(\frac{4m^2-10m+2m-5+9}{2m-5}-2\right)^2-2\)
\(=\left(2m+1+\frac{9}{2m-5}-2\right)^2-2\)
\(=\left(2m-1+\frac{9}{2m-5}\right)^2-2\)
Để P là số nguyên
=> \(\frac{9}{2m-5}\)là số nguyên
=> \(2m-5\in\left\{\pm1;\pm3;\pm9\right\}\)
=> \(m\in\left\{-2;1;2;3;4;7\right\}\)
Kết hợp với ĐK
=> \(m\in\left\{1;2;3;4;7\right\}\)
Vậy \(m\in\left\{1;2;3;4;7\right\}\)
\(2x^2+3mx-\sqrt{2}=0\)
Phương trình có 2 nghiệm phân biệt <=> \(\Delta=\left(3m\right)^2-4\cdot2\cdot\left(\sqrt{2}\right)>0\)
<=> \(9m^2+3\sqrt{2}>0\)(luôn đúng)
=> PT có 2 nghiệm phân biệt x1;x2 với mọi m \(\hept{\begin{cases}x_1+x_2=\frac{-3m}{2}\\x_1x_2=\frac{-\sqrt{2}}{2}\end{cases}}\)
\(M=\left(x_1-x_2\right)^2+\left(\frac{1+x_1^2}{x_1}-\frac{1+x_2^2}{x_2}\right)\)
\(=x_1^2+x_2^2-2x_1x_2+\left[\frac{x_2\left(1+x_1^2\right)-x_1\left(1+x_2^2\right)}{x_1x_2}\right]^2\)
\(=\left(x_1+x_2\right)^2-4x_1x_2+\frac{\left(x_2+x_1+x_1^2x_2-x_1x_2^2\right)^2}{\left(x_1x_2\right)^2}\)
\(=\left(\frac{-3m}{2}\right)^2-4\cdot\left(\frac{\sqrt{2}}{2}\right)+\frac{\left(x_2-x_1\right)^2\cdot\left(1+x_1x_2\right)^2}{\left(x_1x_2\right)^2}\)
\(=\frac{9m^2}{4}+2\sqrt{2}+\frac{\left(\frac{9m^2}{4}+2\sqrt{2}\right)\left(1+\frac{-\sqrt{2}}{2}\right)^2}{\left(\frac{-\sqrt{2}}{2}\right)^2}\)
\(=\frac{9m^2}{4}+2\sqrt{2}+\left(\frac{9m^2}{4}+2\sqrt{2}\right)\left(3-2\sqrt{2}\right)\)
\(=\frac{9m^2}{4}\left(4-2\sqrt{2}\right)+2\sqrt{2}\left(4-2\sqrt{2}\right)\ge2\sqrt{2}\left(4-2\sqrt{2}\right)\ge8\sqrt{2}-8\)
Dấu "=" xảy ra <=> m=0
\(ac< 0\Rightarrow\) phương trình luôn có 2 nghiệm với mọi m
Theo Viet: \(\left\{{}\begin{matrix}x_1+x_2=-\frac{3m}{2}\\x_1x_2=-\frac{\sqrt{2}}{2}\end{matrix}\right.\)
\(M=\left(x_1-x_2\right)^2+\left(x_1-x_2-\frac{x_1-x_2}{x_1x_2}\right)^2\)
\(=\left(x_1-x_2\right)^2+\left(x_1-x_2\right)^2\left(1-\frac{1}{x_1x_2}\right)^2\)
\(=\left(x_1-x_2\right)^2+\left(3+2\sqrt{2}\right)\left(x_1-x_2\right)^2\)
\(=\left(4+2\sqrt{2}\right)\left(x_1-x_2\right)^2\)
\(\Rightarrow\frac{M}{4+2\sqrt{2}}=\left(x_1-x_2\right)^2=\left(x_1+x_2\right)^2-4x_1x_2\)
\(=\frac{9m^2}{4}+2\sqrt{2}\ge2\sqrt{2}\)
\(\Rightarrow M\ge2\sqrt{2}\left(4+2\sqrt{2}\right)=8+8\sqrt{2}\)
Dấu "=" xảy ra khi \(m=0\)
Theo định lý Viet ta có: \(\left\{{}\begin{matrix}x_1+x_2+x_3=m+2\\x_1x_2+x_1x_3+x_2x_3=3m\\x_1x_2x_3=1\end{matrix}\right.\)
\(P=x_1^2+x_2^2+x_3^2=\left(x_1+x_2+x_3\right)^2-2\left(x_1x_2+x_1x_3+x_2x_3\right)\)
\(P=\left(m+2\right)^2-6m=m^2-2m+4\)
\(P=\left(m-1\right)^2+3\ge3\)
\(\Rightarrow P_{min}=3\) khi \(m=1\)
ta thấy pt luôn có no . Theo hệ thức Vi - ét ta có:
x1 + x2 = \(\dfrac{-b}{a}\) = 6
x1x2 = \(\dfrac{c}{a}\) = 1
a) Đặt A = x1\(\sqrt{x_1}\) + x2\(\sqrt{x_2}\) = \(\sqrt{x_1x_2}\)( \(\sqrt{x_1}\) + \(\sqrt{x_2}\) )
=> A2 = x1x2(x1 + 2\(\sqrt{x_1x_2}\) + x2)
=> A2 = 1(6 + 2) = 8
=> A = 2\(\sqrt{3}\)
b) bạn sai đề
Theo vi-et thì ta có:
\(\hept{\begin{cases}x_1+x_2=\frac{3a-1}{2}\\x_1x_2=-1\end{cases}}\)
Từ đây ta có:
\(\left(x_1-x_2\right)^2=\left(x_1+x_2\right)^2-4x_1x_2=\left(\frac{3a-1}{2}\right)^2-4.1=\left(\frac{3a-1}{2}\right)^2-4\)
Theo đề bài thì
\(P=\frac{3}{2}.\left(x_1-x_2\right)^2+2\left(\frac{x_1-x_2}{2}+\frac{1}{x_1}-\frac{1}{x_2}\right)^2\)
\(=\frac{3}{2}.\left(x_1-x_2\right)^2+2.\left(x_1-x_2\right)^2\left(\frac{1}{2}-\frac{1}{x_1x_2}\right)^2\)
\(=\left(x_1-x_2\right)^2\left(\frac{3}{2}+2.\left(\frac{1}{2}-\frac{1}{x_1x_2}\right)^2\right)\)
\(=\left(\left(\frac{3a-1}{2}\right)^2-4\right)\left(\frac{3}{2}+2.\left(\frac{1}{2}+1\right)^2\right)\)
\(=6\left(\left(\frac{3a-1}{2}\right)^2-4\right)\ge6.4=24\)
Dấu = xảy ra khi \(a=\frac{1}{3}\)
Theo Viet ta có \(\left\{{}\begin{matrix}x_1+x_2=-\frac{3m}{2}\\x_1x_2=-\frac{\sqrt{2}}{2}\end{matrix}\right.\)
\(P=\left(x_1+x_2\right)^2-4x_1x_2+\left(\frac{x_1+x_2+x_1x_2\left(x_1+x_2\right)}{x_1x_2}\right)^2\)
\(P=\frac{9m^2}{4}+2\sqrt{2}+\left(\frac{-\frac{3m}{2}-\frac{\sqrt{2}}{2}\left(-\frac{3m}{2}\right)}{-\frac{\sqrt{2}}{2}}\right)^2\)
\(P=\frac{9m^2}{4}+2\sqrt{2}+\left(\frac{27-8\sqrt{2}}{4}\right)m^2\)
\(P=\left(\frac{18-9\sqrt{2}}{2}\right)m^2+2\sqrt{2}\ge2\sqrt{2}\)
\(\Rightarrow P_{min}=2\sqrt{2}\) khi \(m=0\)