4x×(x-1/5)=2/5
x(x-1/5)=1/10
xx-x/5+1/100=1/10+1/100
(x-1/10)(x-1/10)=11/100
x-1/10=根号11/10 x=（根号11+1)/10
x-1/10=-根号11/10 x=（1-根号11)/10