# I can't find the other two solutions to this equation.

As an exercise, I have to use Cardano's formula $$x^3 = px + q$$ $$x = \sqrt{\frac{q}{2}+\sqrt{\frac{q^2}{4} - \frac{p^3}{27}}} + \sqrt{\frac{q}{2}-\sqrt{\frac{q^2}{4} - \frac{p^3}{27}}}$$ to solve the equation $x^3 = 15x+4.$ I finally get $$x = \sqrt{2 + 11i} + \sqrt{2 - 11i}$$ $$x = 4$$ but I have no idea how to find the other two solutions to the equation, and the procedures I've found googling are not employing Cardano's formula.

• How did you prove that $\sqrt{2 + 11i} + \sqrt{2 - 11i}=4$?
Since the degree is 3, if you have a solution, you can make a factorisation and have a second degree equation to solve : \begin{align*} x^3&=15x+4 \tag{1}\\ 4^3&=15\times 4 +4\tag{2} \end{align*} with (1)-(2) : $$x^3-4^3=15x-15\times 4$$ so (with $a^3-b^3=(a-b)(a^2+ab+b^2)$ ) $$(x-4)(x^2 + 4x +16)= 15(x-4) \quad \Leftrightarrow \quad (x-4)(x^2 + 4x +16-15)= 0$$ and you just have to solve then $$x^2 +4x +1$$ to find the others solutions : $$x=-2\pm \sqrt{3}.$$