(X²-1）²-5（X²-1）+4=0
(X²-1-4）（X²-1-1）=0
(X²-5)(X²-2)=0
(X-√5)(X+√5)(X-√2)(X+√2)=0
X1=√5 X2=-√5 X3=√2 X4=-√2