Question : If $2x+\frac{2}{x}=3$, then the value of $x^{3}+\frac{1}{x^{3}}+2$ is:
Option 1: $\frac{3}{4}$
Option 2: $\frac{4}{5}$
Option 3: $\frac{5}{8}$
Option 4: $\frac{7}{8}$
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Correct Answer: $\frac{7}{8}$
Solution : Given: $2x+\frac{2}{x}=3$ $2(x+\frac{1}{x})=3$ $(x+\frac{1}{x})=\frac{3}{2}$ Cubing both sides, we get: $(x+\frac{1}{x})^3=(\frac{3}{2})^3$ ⇒ $x^3+\frac{1}{x^3}+3×x×\frac{1}{x}(x+\frac{1}{x})=\frac{27}{8}$ ⇒ $x^3+\frac{1}{x^3}+3×\frac{3}{2}=\frac{27}{8}$ ⇒ $x^3+\frac{1}{x^3}=\frac{27}{8}–\frac{9}{2}$ ⇒ $x^3+\frac{1}{x^3}=–\frac{9}{8}$ To get the value of $x^{3}+\frac{1}{x^{3}}+2$, we need to add 2 both sides. Thus, $x^{3}+\frac{1}{x^{3}}+2 = –\frac{9}{8}+2$ $\therefore x^{3}+\frac{1}{x^{3}}+2 = \frac{7}{8}$ Hence, the correct answer is $\frac{7}{8}$.
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