已知4x+5y-7z=0 x+2y-z=0,且xyz≠0,试求x²+2y²-6z²/2x²-9y²-4z²的值.

4x+5y-7z=0 .1x+2y-z=0.22式*4-1式得3y+3z=0y+z=0y=-z.3将3式代入2式得x+2*(-z)-z=0x-3z=0x=3z(x²+2y²-6z²)/(2x²-9y²-4z²)=[(3z)²+2(-z)²-6z²]/[2(3z)²-9(-z)
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