2x平方-4x+1=0
2x²-4x+1=0
x²-2x+1=1/2
(x-1)²=1/2
x=1+√2/2 x=1-√2/2
x1+x2= （1+√2/2+1-√2/2）=2
x1x2=（1+√2/2)*(1-√2/2）
=1-(√2/2）²
=1-1/2
=1/2