7x^2-1/5=0
25x^3-10x^2+x=0
7x^2=1/5
x^2=1/35
x=+-корень из 1/35
x(25x^2-10x+1)=0
x=0 25x^2-10x+1=0
x(1-2)=(5+-корень(25-25)) /25
x=5/25
x=0.2