首先確定一下取值范圍
y>0,x>0,y-x>0
ln(y-x)=lny-lnx
=ln(y/x)
y-x=y/x
(1-1/x)y=x
y=x²/(x-1)
1.求導(dǎo)
y'=[2x(x-1)-x²]/(x-1)²
=(x²-2x)/(x-1)²
令y'=0
x=0（舍去）或2
最小值在x=2時取得,為y=2²/(2-1)=4
2.基本不等是
y=x²/(x-1)
=x+1+1/(x-1)
=x-1+1/(x-1)+2
≥2+2
=4
當x-1=1/(x-1),即x=2時等號成立.
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