Répondre :

( x² -x2)-(x² -x1) x (x²+ x2 )
=x^4-(x2)²-(x²+x1)(x²+ x2 )
=x^4-(x2)²-x^4-(x1+x2)x²-x1x2
=-(x2)²-(x1+x2)x²-x1x2
=-(x2)²+b/a x²-c/a

x1=(-b-rac(D))/(2a)
x2=(-b+rac(D))/(2a)