Question 7

If $$(x+\frac{1}{x})^{2}$$=3 then the value $$x^{3}$$+$$\frac{1}{x^{3}}$$ is

Solution

Given,     $$\left(x+\ \frac{\ 1}{x}\right)^2=3$$

                $$x+\ \frac{\ 1}{x}=3^{\ \frac{\ 1}{2}}$$

Hence   $$x^3+\ \frac{\ 1}{x^3}=\left(x+\ \frac{\ 1}{x}\right)^3-3.x.\ \frac{\ 1}{x}\left(x+\ \frac{\ 1}{x}\right)$$

                               $$=\left(3^{\ \frac{\ 1}{2}}\right)^3-3\left(3^{\ \frac{\ 1}{2}}\right)$$

                                $$= 3^{\ \frac{\ 3}{2}}-3^{\ \frac{\ 3}{2}}$$

                                $$= 0$$


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