Question : If $x^2-\sqrt{7} x+1=0$, then $\left(x^3+x^{-3}\right)=?$
Option 1: $4 \sqrt{7}$
Option 2: $3 \sqrt{7}$
Option 3: $10 \sqrt{7}$
Option 4: $7 \sqrt{7}$
Correct Answer: $4 \sqrt{7}$
Solution : Given: $x^2-\sqrt{7} x+1=0$ Dividing both sides by $x$, we get: ⇒ $x-\sqrt{7}+\frac{1}{x}=0$ ⇒ $x+\frac{1}{x}=\sqrt{7}$ Cubing both sides, we get: ⇒ $(x+\frac{1}{x})^3=(\sqrt{7})^3$ ⇒ $x^3+\frac{1}{x^3}+3(x\times\frac{1}{x})(x+\frac{1}{x})=(\sqrt{7})^3$ ⇒ $x^3+\frac{1}{x^3}+3\times\sqrt7=7\sqrt{7}$ ⇒ $x^3+\frac{1}{x^3}=4\sqrt{7}$ Hence, the correct answer is $4\sqrt{7}$.
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Question : If $\sqrt{x}+\frac{1}{\sqrt{x}}=3, x>0$, then $x^2\left(x^2-47\right)=?$
Option 1: $0$
Option 2: $2$
Option 3: $-2$
Option 4: $-1$
Question : If $x^2-3 x+1=0$, then the value of $\left(x^4+\frac{1}{x^2}\right) \div\left(x^2+1\right)$ is:
Option 1: 5
Option 2: 6
Option 3: 9
Option 4: 7
Question : If $x^2+\frac{1}{x^2}=\frac{7}{4}$ for $x>0$; then what is the value of $x+\frac{1}{x}$?
Option 1: $2$
Option 2: $\frac{\sqrt{15}}{2}$
Option 3: $\sqrt{5}$
Option 4: $\sqrt{3}$
Question : If $\left(5 \sqrt{5} x^3-3 \sqrt{3} y^3\right) \div(\sqrt{5} x-\sqrt{3} y)=\left(A x^2+B y^2+C x y\right)$, then the value of $(3 A+B-\sqrt{15} C)$ is:
Option 1: 8
Option 2: 5
Option 3: 3
Option 4: 12
Question : If $x^2-5 x+1=0$, then the value of $\left(x^4+\frac{1}{x^2}\right) \div\left(x^2+1\right)$ is:
Option 1: 21
Option 2: 22
Option 3: 25
Option 4: 24
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