已知x-y=6,xy=-8,
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已知x-y=6,xy=-8,
(1)求x2+y2的值;
(2)求代数式
(x+y+z)
(1)求x2+y2的值;
(2)求代数式
1 |
2 |
(1)∵x-y=6,xy=-8,
∴(x-y)2=x2+y2-2xy,
∴x2+y2=(x-y)2+2xy=36-16=20;
(2)∵
1
2(x+y+z)2+
1
2(x-y-z)(x-y+z)-z(x+y),
=
1
2(x2+y2+z2+2xy+2xz+2yz)+
1
2[(x-y)2-z2]-xz-yz,
=
1
2x2+
1
2y2+
1
2z2+xy+xz+yz+
1
2x2+
1
2y2-xy-
1
2z2-xz-yz,
=x2+y2,
又∵x2+y2=20,
∴原式=20.
∴(x-y)2=x2+y2-2xy,
∴x2+y2=(x-y)2+2xy=36-16=20;
(2)∵
1
2(x+y+z)2+
1
2(x-y-z)(x-y+z)-z(x+y),
=
1
2(x2+y2+z2+2xy+2xz+2yz)+
1
2[(x-y)2-z2]-xz-yz,
=
1
2x2+
1
2y2+
1
2z2+xy+xz+yz+
1
2x2+
1
2y2-xy-
1
2z2-xz-yz,
=x2+y2,
又∵x2+y2=20,
∴原式=20.
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