x²-2x+1=0
除以x得x-2+1/x=0
x+1/x=3
﹙x+1/x)²=9=x²+2+1/x²
∴x²+1/x²=7
x^4+1/x^4=(x²+1/x²﹚²-2
=7²-2
=47