X+y=9 xy=-10 система квадратных уравнений

X + y = 9
xy = - 10

x = 9 - y
y (9 - y ) + 10 = 0 *

9y - y^2 + 10 = 0 
- y^2 + 9y +10 = 0 
y^2 - 9y - 10 = 0 
D = 81 + 40 = 121 = 11^2
y1 = ( 9 + 11)/2 = 20/2 = 10;
y2 = ( 9 - 11)/2 = - 2/2 = - 1;

y1 = 10
x1 = 9 - y1 = 9 - 10 = - 1;

y2 = - 1
x1 = 9 - y2 = 9 + 1 = 10 

ответ
( - 1; 10)
( 10; - 1)