已知(1):x+2y-9z=0,(2):x-2y-5z=0,xyz不等于0求(2x^2+3y^2+7z^2)/(x^2-4y^2+9z^2)的值.
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已知(1):x+2y-9z=0,(2):x-2y-5z=0,xyz不等于0求(2x^2+3y^2+7z^2)/(x^2-4y^2+9z^2)的值.
已知(1):x+2y-9z=0,(2):x-2y-5z=0,xyz不等于0求(2x^2+3y^2+7z^2)/(x^2-4y^2+9z^2)的值.
已知(1):x+2y-9z=0,(2):x-2y-5z=0,xyz不等于0求(2x^2+3y^2+7z^2)/(x^2-4y^2+9z^2)的值.
(1):x+2y-9z=0
(2):x-2y-5z=0
(1)+(2)得
2x-14z=0
x=7z
(2)-(1)得
-4y+4z=0
y=-z
于是
原式
=[2(7z)²+3(-z)²+7z²]/[(7z)²-4(-z)²+9z²]
=(98+3+7)z²/(49-4+9)z²
=108z²/54z²
=2
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