x²-5x+1=0x-5+1/x=0x+1/x=5(x+1/x)²=25x²+2+1/x²=25x²-2+1/x²=21(x-1/x)²=21x-1/x=正负根号21x²-1/x²=(x+1/x)(x-1/x)=正负5根号21
