已知x+y=-5,xy=3,则x²+y²的值是19.为啥~还有一道.差不多的若x+y=3,xy=1,求x²+y²的值

已知x+y=-5,xy=3,则x²+y²的值是19.为啥~还有一道.差不多的若x+y=3,xy=1,求x²+y²的值
已知x+y=-5,xy=3,则x²+y²的值是19.为啥~
还有一道.差不多的
若x+y=3,xy=1,求x²+y²的值

已知x+y=-5,xy=3,则x²+y²的值是19.为啥~还有一道.差不多的若x+y=3,xy=1,求x²+y²的值
若x+y=-5,xy=3
则x²+y²
=x²+y²+2xy-2xy
=(x+y)²-2xy
=(-5)²-2*3
=19
同理：若x+y=3,xy=1,
则x²+y²=(x+y)²-2xy=3²-2*1=7

x+y=-5,xy=3,则x²+y²=(x+y)^2 =x^2 + y^2 + 2xy = 2
=>x^2 + y^2 = 25 - 2xy = 25 - 6 =19
x+y=3,xy=1,求x²+y²
(x+y)^2 = 9
x^2 +y^2 + 2xy = 9
=> x^2 + y^2 = 9 - 2 = 7

（x+y）²=9
x²+2xy+y²=9
2xy=2
所以x²+y²=9-2=7

x²+y²=(x+y)²-2xy=3²-2=7