原始除以xx+1/x=3把上式平方x^2+1/x^2+2=9x^2+1/x^2=7
(x^2-3x+1)/x=0/x得到x-3+1/x=0 x+1/x=3所以(x+1/x)^2=9 x^2+2+1/x^2=9(完全平方公式)所以x^2+1/x^2=7