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4x-y-8z=0 (1)
x+2y-8z=0 (2)
(1)-(2)
3x-3y=0
y=x

(1)*2+(2)
8x-16z+x-8z=0
9x=24z
z=3x/8

(2x^2+3y^2+11z^2)/(7x^2+8y^2+26z^2)
=(2x^2+3x^2+11*9/64x^2)/(7x^2+8x^2+26*9/64x^2)
=(419/64x^2)/(1194/64x^2)
=419/1194

4x-y-8z=0(1式)
x+2y-8z=0（2式）

1式×2+2式
9x=24z
z=3x/8

1式-2式
x=y

(2x^2+3y^2+11z^2)/(7x^2+8y^2+26z^2)
=(2x^2+3y^2+11*9/64x^2)/(7x^2+8x^2+26*9/64z^2)
=419/1194

4x-y-8z=0
x+2y-8z=0
将z看作是已知数，可以求解x,y
4x-y=8z
x+2y=8z
解得x=8z/3，y=8z/3，即x=y=8z/3

(2x²+3y²+11z²)/(7x²+8y²+26z²)
=(5x²+11z²)/(15x²+26z²)
=[5(8z/3)²+11z²]/[15(8z/3)²+26z²]
=(320z²+99z²)/(960z²+234z²)
=419z²/1194z²
=419/1194

4x-y=8z
x+2y=8z
解得:
x=y=8z/3

(2x^2+3y^2+11z^2)/(7x^2+8y^2+26z^2)
=(2*64/9