若a+1/a=3,则a-1/a=(?);已知x^2-2xy=47,3xy+4y^2=-19,则3x^2+8y^2=(?)
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若a+1/a=3,则a-1/a=(?);已知x^2-2xy=47,3xy+4y^2=-19,则3x^2+8y^2=(?);已知a:b:c=2:3:4,a+b+c=27,则
a-2b-2c=(?)
a-2b-2c=(?)
a+1/a=3
所以1/a=2
a=1/2
a-1/a=-2/3
x^2-2xy=47,3xy+4y^2=-19
3x^2-6xy=141(1),6xy+8y^2=-38(2)
(1)+(2)
3x^2+8y^2=103
设a=2X,b=3x,c=4x
2x+3x+4x=27
x=3
a=6,b=9,c=12
a-2b-2c=-36
所以1/a=2
a=1/2
a-1/a=-2/3
x^2-2xy=47,3xy+4y^2=-19
3x^2-6xy=141(1),6xy+8y^2=-38(2)
(1)+(2)
3x^2+8y^2=103
设a=2X,b=3x,c=4x
2x+3x+4x=27
x=3
a=6,b=9,c=12
a-2b-2c=-36
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若a+1/a=3,则a-1/a=(?);已知x^2-2xy=47,3xy+4y^2=-19,则3x^2+8y^2=(?)
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