①x^2-4x+1=0
x^2-4x+4-4+1=0
(x-2)^2-3=0
(x-2+√3)(x-2-√3)=0
x=2-√3或x=2+√3
②x(x-1)/2=21
x(x-1)=42
x²-x-42=0
(x-7)(x+6)=0
x=7或x=-6