\(G^2=x^2\left(1-x^2\right)=-x^4+x^2=-\left(x^2-\frac{1}{2}\right)^2+\frac{1}{4}\le\frac{1}{4}\)
\(\Rightarrow-\frac{1}{2}\le G\le\frac{1}{2}\Rightarrow G_{min}=-\frac{1}{2}\) khi \(x=-\frac{\sqrt{2}}{2}\)
Nếu được phép sử dụng AM-GM thì:
\(G^2=x^2\left(1-x^2\right)\le\frac{1}{4}\left(x^2+1-x^2\right)^2=\frac{1}{4}\Rightarrow G\ge-\frac{1}{2}\)