`x^4 + 4 = 5x^2`
`⇔ x^4 - 5x^2 + 4 = 0`
Đặt `x^2 = t(t \ge 0)`
`⇔ t^2 - 5t + 4 = 0`
`⇔ (t-1)(t-4) = 0`
`⇔`\(\left[ \begin{array}{l}t-1=0\\t-4=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}t=1\\t=4\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x^2=4\\x^2=1\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=\pm2\\x=\pm1\end{array} \right.\)
Vậy `S = {\pm2,\pm1}`
