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=（?)
问题描述：
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=（?)
答
-20.25
答
a+1/a=3所以1/a=2a=1/2a-1/a=-2/3x^2-2xy=47,3xy+4y^2=-193x^2-6xy=141（1）,6xy+8y^2=-38（2）（1）+（2）3x^2+8y^2=103设a=2X,b=3x,c=4x2x+3x+4x=27x=3a=6,b=9,c=12a-2b-2c=-36