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If $\alpha, \beta$ are the roots of the equations $x^{2}-2 x-1=$ $0,$ then what is the value of $\alpha^{2} \beta^{-2}+\alpha^{-2} \beta^{2}$
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The correct answer is:
34
Since, \(\alpha\) and \(\beta\) are the roots of the equation \(x^{2}-2 x-1=0\), then
Sum of roots, \(\alpha+\beta=2\) and
product of the roots \(\alpha \beta=-1\)
Since, \((\alpha+\beta)=\alpha^{2}+\beta^{2}+2 \alpha \beta\)
\(\Rightarrow 4=\alpha^{2}+\beta^{2}-2\)
\(\Rightarrow \alpha^{2}+\beta^{2}=6\)
Now, \(\alpha^{2} \beta^{-2}+\alpha^{-2} \beta^{2}=\frac{\alpha^{2}}{\beta^{2}}+\frac{\beta^{2}}{\alpha^{2}}=\frac{\alpha^{4}+\beta^{4}}{(\alpha \beta)^{2}}\)
\(\Rightarrow\left(\alpha^{2}+\beta^{2}\right)^{2}=6^{2}\)
\(\Rightarrow \alpha^{4}+\beta^{4}+2 \alpha^{2} \beta^{2}=36\)
\(\Rightarrow \alpha^{4}+\beta^{4}+2=36\)
\(\Rightarrow \alpha^{4}+\beta^{4}=34 \ldots . .(\mathrm{i})\)
\(\Rightarrow \frac{\alpha^{4}+\beta^{4}}{(\alpha \beta)^{2}}=\frac{34}{(-1)^{2}}=34\)
[Putting value of \(\alpha^{4}+\beta^{4}=34\) from Equation (i) \(]\)
Sum of roots, \(\alpha+\beta=2\) and
product of the roots \(\alpha \beta=-1\)
Since, \((\alpha+\beta)=\alpha^{2}+\beta^{2}+2 \alpha \beta\)
\(\Rightarrow 4=\alpha^{2}+\beta^{2}-2\)
\(\Rightarrow \alpha^{2}+\beta^{2}=6\)
Now, \(\alpha^{2} \beta^{-2}+\alpha^{-2} \beta^{2}=\frac{\alpha^{2}}{\beta^{2}}+\frac{\beta^{2}}{\alpha^{2}}=\frac{\alpha^{4}+\beta^{4}}{(\alpha \beta)^{2}}\)
\(\Rightarrow\left(\alpha^{2}+\beta^{2}\right)^{2}=6^{2}\)
\(\Rightarrow \alpha^{4}+\beta^{4}+2 \alpha^{2} \beta^{2}=36\)
\(\Rightarrow \alpha^{4}+\beta^{4}+2=36\)
\(\Rightarrow \alpha^{4}+\beta^{4}=34 \ldots . .(\mathrm{i})\)
\(\Rightarrow \frac{\alpha^{4}+\beta^{4}}{(\alpha \beta)^{2}}=\frac{34}{(-1)^{2}}=34\)
[Putting value of \(\alpha^{4}+\beta^{4}=34\) from Equation (i) \(]\)
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