x+1/x=2√2 平方x^2+2+1/x^2=8x^2+1/x^2=6x^2-2+1/x^2=6-2=4(x-1/x)^2=4x-1/x=±2
(x+1/x)^2=8两端-2，再配方。(x-1/x)^2=6开方即可。
