31.3 a) 9г²-20-21-0: 6) 7x²+6x-1=0;

31.3

a) $$9x^2-20x-21=0$$

$$D = (-20)^2 - 4 \cdot 9 \cdot (-21) = 400 + 756 = 1156 = 34^2$$

$$x_1 = \frac{20 + 34}{2 \cdot 9} = \frac{54}{18} = 3$$

$$x_2 = \frac{20 - 34}{2 \cdot 9} = \frac{-14}{18} = -\frac{7}{9}$$

б) $$7x^2+6x-1=0$$

$$D = 6^2 - 4 \cdot 7 \cdot (-1) = 36 + 28 = 64$$

$$x_1 = \frac{-6 + 8}{2 \cdot 7} = \frac{2}{14} = \frac{1}{7}$$

$$x_2 = \frac{-6 - 8}{2 \cdot 7} = \frac{-14}{14} = -1$$

Ответ: a) $$x_1=3$$, $$x_2=-\frac{7}{9}$$, б) $$x_1=\frac{1}{7}$$, $$x_2=-1$$