x+1/x=3(x+1/x)^2=9x^2+2+1/(x^2)=9x^2+1/(x^2)=7(x-1/x)^2=x^2-2+1/(x^2)=7-2=5x-1/x=±√5
﹙x－1／x﹚²＝﹙x＋1／x﹚²－4＝3²－4=5x－1／x=±√5
