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Chắc là \(q\left(x\right)=x^2-4????\)
\(f\left(2\right)=2^5+2^2+1=37\) ; \(f\left(-2\right)=-27\)
Do \(f\left(x\right)\) có 5 nghiệm nên f(x) có dạng:
\(f\left(x\right)=\left(x-x_1\right)\left(x-x_2\right)\left(x-x_3\right)\left(x-x_4\right)\left(x-x_5\right)\)
\(\Rightarrow f\left(2\right)=\left(2-x_1\right)\left(2-x_2\right)\left(2-x_3\right)\left(2-x_4\right)\left(2-x_5\right)=37\)
\(f\left(-2\right)=\left(-2-x_1\right)\left(-2-x_2\right)\left(-2-x_3\right)\left(-2-x_4\right)\left(-2-x_5\right)=-27\)
\(\Rightarrow\left(2+x_1\right)\left(2+x_2\right)\left(2+x_3\right)\left(2+x_4\right)\left(2+x_5\right)=27\)
\(A=\left(x_1^2-4\right)\left(x^2_2-4\right)\left(x_3^2-4\right)\left(x_4^2-4\right)\left(x^2_5-4\right)\)
\(A=-\left(2-x_1\right)\left(2-x_2\right)\left(2-x_3\right)\left(2-x_4\right)\left(2-x_5\right)\left(2+x_1\right)\left(2+x_2\right)\left(2+x_3\right)\left(2+x_4\right)\left(2+x_5\right)\)
\(A=-37.27=-999\)
Theo Vi-ét cho 3 số (chứng minh bằng hệ số bất định)
\(\hept{\begin{cases}x_1+x_2+x_3=0\\x_1x_2+x_2x_3+x_1x_3=-3\\x_1x_2x_3=-1\end{cases}}\)
\(A=\frac{1+2x_1}{1+x_1}+\frac{1+2x_2}{1+x_2}+\frac{1+2x_3}{1+x_3}\)
\(=3+\frac{x_1}{1+x_1}+\frac{x_2}{1+x_2}+\frac{x_3}{1+x_3}\)
\(=3+\frac{x_1\left(1+x_2\right)\left(1+x_3\right)+x_2\left(1+x_1\right)\left(1+x_3\right)+x_3\left(1+x_1\right)\left(1+x_2\right)}{\left(1+x_1\right)\left(1+x_2\right)\left(1+x_3\right)}\)
\(=3+\frac{x_1\left(1+x_2+x_3+x_2x_3\right)+x_2\left(1+x_1+x_3+x_1x_3\right)+x_3\left(1+x_1+x_2+x_1x_2\right)}{\left(1+x_1+x_2+x_1x_2\right)\left(1+x_3\right)}\)
\(=3+\frac{\left(x_1+x_2+x_3\right)+2\left(x_1x_2+x_2x_3+x_3x_1\right)+3x_1x_2x_3}{1+x_1+x_2+x_3+x_1x_2+x_1x_3+x_2x_3+x_1.x_2.x_3}\)
\(=3+\frac{0+2.\left(-3\right)+3.\left(-1\right)}{1+0-3-1}\)
\(=6\)
Do x1 là một nghiệm của đa thức f(x) nên ta có: \(x_1^3-3x_1+1=0\)
\(\Leftrightarrow\)\(\left(x_1+1\right)\left(x_1^2-x_1+1\right)=3x_1\)\(\Leftrightarrow\)\(x_1+1=\frac{3x_1}{x_1^2-x_1+1}\)
Có: \(A==\frac{1+2x_1}{1+x_1}+\frac{1+2x_2}{1+x_2}+\frac{1+2x_3}{1+x_3}=3+\left(\frac{x_1}{1+x_1}+\frac{x_2}{1+x_2}+\frac{x_3}{1+x_3}\right)\)
\(A=3+\left(\frac{x_1\left(x_1^2-x_1+1\right)}{3x_1}+\frac{x_2\left(x^2_2-x_2+1\right)}{3x_2}+\frac{x_3\left(x_3^2-x_3+1\right)}{3x_3}\right)\)
\(A=3+\frac{\left(x_1^2+x_2^2+x_3^2\right)-\left(x_1+x_2+x_3\right)+3}{3}\)
\(A=3+\frac{\left(x_1+x_2+x_3\right)^2-2\left(x_1x_2+x_2x_3+x_3x_1\right)-\left(x_1+x_2+x_3\right)+3}{3}\)
Đến đây theo Vi-et bậc 3
\(\hept{\begin{cases}x_1+x_2+x_3=0\\x_1x_2+x_2x_3+x_3x_1=-3\end{cases}}\)
Với n=2
=> \(x_1+\frac{1}{x_1}=x_2+\frac{1}{x_2}\)
\(\Rightarrow x_1-x_2=\frac{1}{x_1}-\frac{1}{x_2}\)
\(\Rightarrow\left(x_1-x_2\right)-\frac{x_1-x_2}{x_1x_2}=0\)
\(\Rightarrow\left(x_1-x_2\right)\left(1-\frac{1}{x_1x_2}\right)=0\)
\(\Rightarrow\orbr{\begin{cases}x_1-x_2=0\\1-\frac{1}{x_1x_2}=0\end{cases}\Rightarrow\orbr{\begin{cases}x_1=x_2\\x_1x_2=1\end{cases}}}\)
*) n=k
=> \(x_1+\frac{1}{x_1}=x_2+\frac{1}{x_2}=...=x_k+\frac{1}{x_k}\)
thì \(x_1=x_2=x_3=...=x_k\)hoặc \(\left|x_1x_2...x_k\right|=0\)
Với n=k+1
=> \(x_1+\frac{1}{x_1}=x_2+\frac{1}{x_2}=x_3+\frac{1}{x_3}=...x_{k+1}+\frac{1}{x_1}\)
=> \(x_1+\frac{1}{x_2}=x_2+\frac{1}{x_3}=....=x_k+\frac{1}{x_{k+1}}=x_{k+1}+\frac{1}{x_1}\)
\(\Rightarrow x_{k-1}+\frac{1}{x_k}=x_k+\frac{1}{x_1}=x_{k+1}+\frac{1}{x_1}\)
\(\Rightarrow x_k-x_{k+1}=0\)
\(\Rightarrow x_k=x_{k+1}\)
\(\Rightarrow x_1=x_2=...=x_k=x_{k+1}\)
Lời giải sẽ dài lắm nhé
x1,x2 là hai nghiệm của \(P(x)\)nên :
\(P(x_1)=ax_1^2+bx_1+c=0\) \((1)\)
\(P(x_2)=ax^2_2+bx^2+c=0\)
\(P(x_1)-P(x_2)=a\left[x^2_1-x^2_2\right]+b\left[x_1-x_2\right]=0\)
\(a\left[x_1+x_2\right]\left[x_1-x_2\right]+b\left[x_1-x_2\right]=0\)
\(\left[x_1-x_2\right]\left[a\left\{x_1+x_2\right\}+b\right]=0\)
Vì x1 \(\ne\)x2 nên x1 - x2 \(\ne\)0 do đó
\(a\left[x_1+x_2\right]+b=0\Rightarrow b=-a\left[x_1+x_2\right]\) \((2)\)
Thế 2 vào 1 ta được :
\(ax^2_1-a\left[x_1+x_2\right]\cdot x_1+c=0\)
\(\Rightarrow c=ax_1\left[x_1+x_2\right]-ax^2_1=ax_1x_2\) \((3)\)
Thế 2 vào 3 vào P\((x)\)ta được :
\(P(x)=ax^2+bx+c=ax^2-ax\left[x_1+x_2\right]+ax_1x_2\)
\(=ax^2-axx_1-axx_2+ax_1x_2=a\left[x^2-xx_1-xx_2+x_1x_2\right]\)
\(=a\left[x\left\{x-x_1\right\}-x_2\left\{x-x_1\right\}\right]=a\left[x-x_1\right]\left[x-x_2\right]\)
Vậy : ....
