FRAC{1}{x}=2\cos \THETA \], Now \[{{x}^{3}}+\frac{1}{{{x}^{3}}}={{\left( x+\frac{1}{x} \right)}^{3}}-3x\frac{1}{x}\left( x+\frac{1}{x} \right)\] = \[{{(2\cos \theta )}^{3}}-3(2\cos \theta )=8{{\cos }^{3}}\theta -6\cos \theta \] = \[2(4{{\cos }^{3}}\theta -3\cos \theta )=2\cos 3\theta \]. Trick: Put \[x=1\] \[\RIGHTARROW \] \[\theta ={{0}^{{}^\CIRC }}\]. Then\[{{x}^{3}}+\frac{1}{{{x}^{3}}}=2=2\cos 3\theta \].