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-
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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.
(1):x+2y-9z=0(2):x-2y-5z=0(1)+(2)得2x-14z=0x=7z(2)-(1)得-4y+4z=0y=-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
=(98+3+7)z²/(49-4+9)z²=108z²/54z²=2
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