x^4-5x^3+5x^3-25x^2-5x^2+25x+6x-30=0
(x-5)(x^3+5x^2-5x+6)=0
(x-5)(x^3+6x^2-x^2-6x+x+6)=0
(x-5)(x+6)(x^2-x+1)=0
Suy ra x-5=0 hay x+6=0 hay x^2-x+1=0
Suy ra x=5 hay x=-6 hay x^2+2x.1/2+1/4+3/4=0
Suy ra x=5 hay x=-6 hay (x+1/2)^2=3/4=0 (vô lý)
Vậy x=5 hay x=-6