`x^4 - 15x^2 + 56 = 0 `
`⇔ x^4 - 8x^2 - 7x^2 + 56 = 0`
`⇔ x^2(x^2 - 8 ) - 7( x^2 - 8 ) =0`
`⇔ ( x^2 - 8 )(x^2 - 7 ) = 0`
`⇔ ( x - \sqrt{8} )( x + \sqrt{8})( x - \sqrt{7} )( x + \sqrt{7} ) = 0 `
`⇔` \(\left[ \begin{array}{l}x=±\sqrt{8}\\x=±\sqrt{7}\end{array} \right.\)
`S = ` { `±\sqrt{8} ; ±\sqrt{7}` }