a) x^4 - 13x^2 + 18x - 5 = 0 .

b) x^4 = 4x +1 .

c)* giải :
( x^2 -1 )^2 + 4( x-1 )^2 = 12(x+1)^2 .
a.
[TEX] x^4 - 13x^2 + 18x - 5 = 0[/TEX]
[TEX]\Leftrightarrow (x^4 - 3x^3 + x^2) + (3x^3 - 9x^2 + 3x) - (5x^2 - 15x + 5) = 0[/TEX]
[TEX]\Leftrightarrow x^2(x^2 - 3x + 1) + 3x(x^2 - 3x + 1) - 5(x^2 - 3x + 1) = 0 [/TEX]
[TEX]\Leftrightarrow (x^2 + 3x - 5)(x^2 - 3x + 1) = 0[/TEX]
[TEX]\Delta = 3^2 - 4.1.(-5) = 29[/TEX]
=>
[TEX]x_1=\frac{-3-\sqrt{29}}{2}[/TEX]
[TEX]x_2=\frac{-3+\sqrt{29}}{2}[/TEX]
[TEX]\Delta = (-3)^2 - 4.1.1 = 5[/TEX]
=>[TEX] x_3 = \frac{3-\sqrt{5}}{2} [/TEX]
[TEX] x_4 = \frac{3+\sqrt{5}}{2} [/TEX]