[tex]\displaystyle -x^2-7x+8=0 \\ a=-1,b=-7,c=8 \\ \Delta=b^2-4ac=7^2-4 \cdot (-1) \cdot 8=49+32=81\ \textgreater \ 0 \\ x_1= \frac{7+ \sqrt{81} }{2 \cdot (-1)} = \frac{7+9}{-2} = \frac{16}{-2} =-8 \\ \\ x_2= \frac{7- \sqrt{81} }{2 \cdot (-1)} = \frac{7-9}{-2} = \frac{-2}{-2} =1 \\ \\ S=\{-8;1\}[/tex]
