x+1/x=2倍根号5两边平方得：x²+2+1/x²=20x²+1/x²=20-2=18x-1/x=±根号(x-1/x)²=±根号(18-2)=±4
x+1/x=2根号5(x+1/x)^2=x^2+2+1/x^2=20 x^2+1/x^2=18(x-1/x)^2=x^2-2+1/x^2=18-2=16x-1/x=±4
