5x^4-8x^2+1=0
x^2 = t ; t>0
5t^2 -8t +1 =0
D =(-8)^2 -4*5*1 =44
√D = -/+ 2√11
t = 1/10 *(8 -/+2√11 )
t1 = (8 - 2√11 )/10 = (4-√11)/5 ; x^2 = (4-√11)/5
x1 = - √(4-√11)/5
x2 = √(4-√11)/5
t2 = (8 + 2√11 )/10 = (4+√11)/5 ; x^2 = (4+√11)/5
x3 = - √(4+√11)/5
x4 = √(4+√11)/5
сумма всех корней (x1+x2)+(x3+x4) = 0+0 =0