（X+x分之1）的平方=9x^2+2+1/x^2=9x^2+1/x^2=7(x-1/x)^2=x^2-2+1/x^2=7-2=5
(x+1/x)²=9(x+1/x)²=9x²+2/x+1/x²=9 两边各减4→x²-2+1/x²=5则（x-1/x)²=5
