x+（1/x）=3
左右平方,二项式公式
(x+(1/x))^2=x^2+2+1/(x^2)=9
x^2+1/(x^2)=7
再平方
(x^2+1/(x^2))^2=x^4+2+1/(x^4)=49
x^4+1/(x^4)=47