How to solve this simple problem So my younger brother asked me to solve a simple problem, but I'm not sure how to solve this. Any help would be much appreciated. 
$$a^3 - 1/ a^3 = 4. $$
Prove that  $$a - 1/a = 1$$

Using  $$\displaystyle \left(a-\frac{1}{a}\right)^3=\left(a^3-\frac{1}{a^3}\right)-3\cdot a\cdot \frac{1}{a}\left(a-\frac{1}{a}\right)$$
So we get $$\displaystyle \left(a-\frac{1}{a}\right)^3=4-3\displaystyle \left(a-\frac{1}{a}\right)\;,$$ Now Put $\displaystyle \left(a-\frac{1}{a}\right)=x$
So we get $$\displaystyle x^3=4-3x\Rightarrow x^3+3x-4=0\Rightarrow (x-1)\cdot (x^2+x+4)=0$$
So we get $x=1$ or $\displaystyle x^2+x+4 =0\Rightarrow x=\frac{-1\pm \sqrt{1-16}}{2}$ (No real values of $x$)
