[tex] - {x}^{2} - 7x + 8 = 0 \: | \times ( - 1)[/tex]
[tex] {x}^{2} + 7x - 8 = 0[/tex]
[tex]a = 1[/tex]
[tex]b = 7[/tex]
[tex]c = - 8[/tex]
[tex]\Delta = {b}^{2} - 4ac[/tex]
[tex]\Delta = {7}^{2} - 4 \times 1 \times ( - 8)[/tex]
[tex]\Delta = 49 + 32[/tex]
[tex]\Delta = 81[/tex]
[tex]x_{1,2}=\frac{-b\pm\sqrt{\Delta}}{2a}[/tex]
[tex]x_{1,2}=\frac{-7\pm\sqrt{81}}{2 \times 1}[/tex]
[tex]x_{1,2}=\frac{-7\pm9}{2}[/tex]
[tex]x_{1}=\frac{-7 + 9}{2} = \frac{2}{2} = 1[/tex]
[tex]x_{2}=\frac{-7 - 9}{2} = - \frac{16}{2} = - 8[/tex]
