已知x3+y3-z3=96,xyz=4,x2+y2+z2-xy+xz+yz=12,则x+y-z=( )
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已知x3+y3-z3=96,xyz=4,x2+y2+z2-xy+xz+yz=12,则x+y-z=( )
A. 6
B. 7
C. 8
D. 9
A. 6
B. 7
C. 8
D. 9
x3+y3-z3+3xyz,
=[(x+y)3-3x2y-3xy2]-z3+3xyz,
=[(x+y)3-z3]-(3x2y+3xy2-3xyz),
=(x+y-z)[(x+y)2+(x+y)z+z2]-3xy(x+y-z),
=(x+y-z)(x2+2xy+y2+xz+yz+z2-3xy),
=(x+y-z)(x2+y2+z2-xy+xz+yz),
∵x3+y3-z3=96,xyz=4,x2+y2+z2-xy+xz+yz=12,
∴96+3×4=12(x+y-z),
解得x+y-z=9.
故选D.
=[(x+y)3-3x2y-3xy2]-z3+3xyz,
=[(x+y)3-z3]-(3x2y+3xy2-3xyz),
=(x+y-z)[(x+y)2+(x+y)z+z2]-3xy(x+y-z),
=(x+y-z)(x2+2xy+y2+xz+yz+z2-3xy),
=(x+y-z)(x2+y2+z2-xy+xz+yz),
∵x3+y3-z3=96,xyz=4,x2+y2+z2-xy+xz+yz=12,
∴96+3×4=12(x+y-z),
解得x+y-z=9.
故选D.
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