若x-y=6,y-z=5,求x²+y²+z²-xy-xz-yz的值.
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若x-y=6,y-z=5,求x²+y²+z²-xy-xz-yz的值.
计算题:
1.19.8²-39.6×40.2+40.2²
2.(2分之1a-3b)(3b-2分之1a)
计算题:
1.19.8²-39.6×40.2+40.2²
2.(2分之1a-3b)(3b-2分之1a)
![若x-y=6,y-z=5,求x²+y²+z²-xy-xz-yz的值.](/uploads/image/z/19710294-6-4.jpg?t=%E8%8B%A5x-y%3D6%2Cy-z%3D5%2C%E6%B1%82x%26sup2%3B%2By%26sup2%3B%2Bz%26sup2%3B-xy-xz-yz%E7%9A%84%E5%80%BC.)
x-y=6
y-z=5
两式相加:
x-z=11
x²+y²+z²-xy-xz-yz
=2(x²+y²+z²-xy-xz-yz)÷2
=(2x²+2y²+2z²-2xy-2xz-2yz)÷2
=(x²-2xy+y²x²-2xz+z²+y²-2yz+z²)÷2
=[(x-y)²+(x-z)²+(y-z)²]÷2
=(36+121+25)÷2
=91
y-z=5
两式相加:
x-z=11
x²+y²+z²-xy-xz-yz
=2(x²+y²+z²-xy-xz-yz)÷2
=(2x²+2y²+2z²-2xy-2xz-2yz)÷2
=(x²-2xy+y²x²-2xz+z²+y²-2yz+z²)÷2
=[(x-y)²+(x-z)²+(y-z)²]÷2
=(36+121+25)÷2
=91
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