x+1/x=3
(x+1/x)^2=3^2
x^2+1/x^2+2=9
x^2+1/x^2=7
(x^2+1/x^2)^2=7^2
x^4+1/x^4+2=49
x^4+1/x^4=47
(^为乘方,x^2为x的平方)