1.已知a+b=3,a+²b²=8,求a²+3ab+b²2.已知x+y+z=5,xy+yz+zx=9,求x²+y²+z²的值²(...的平方,

1.已知a+b=3,a+²b²=8,求a²+3ab+b²2.已知x+y+z=5,xy+yz+zx=9,求x²+y²+z²的值²(...的平方,
1.已知a+b=3,a+²b²=8,求a²+3ab+b²
2.已知x+y+z=5,xy+yz+zx=9,求x²+y²+z²的值
²(...的平方,

1.已知a+b=3,a+²b²=8,求a²+3ab+b²2.已知x+y+z=5,xy+yz+zx=9,求x²+y²+z²的值²(...的平方,
1.已知a+b=3,a²+b²=8
那么ab=[(a+b)²-(a²+b²)]/2=(3²-8)/2=1/2
所以a²+3ab+b²=8+3*1/2=19/2
2.已知x+y+z=5,xy+yz+zx=9
那么(x+y+z)²=x²+y²+z²+2(xy+yz+zx)=x²+y²+z²+18=25
所以x²+y²+z²=7

﹙x+y+z﹚²=5²=25，展开得到2xy+2yz+2zx+x²+y²+z²=25，所以x²+y²+z²=16 微笑