4) 2y^4-5y^2-7=(2y^2-7)(y^2+1)=0|:(y^2+1)≠0
y^2-7/2=(y-sqrt(7/2))(y+sqrt(7/2))=0
y=sqrt(7/2)
y=-sqrt(7/2)
5) x^4-(a^2+9)x^2+9a^2=0
(x^2-9)(x^2-a^2)=(x-3)(x+3)(x-a)(x+a)=0
x=3
x=-3
x=a
x=-a
6) x^4-(9a^2+4)x^2+36a^2=0
(x^2-9a^2)(x^2-4)=(x-3a)(x+3a)(x-2)(x+2)=0
x=3a
x=-3a
x=2
x=-2