det(A(a))=8+ [tex]a^{3} [/tex] +[tex] 2a^{2} [/tex] - [tex]2 a^{2} [/tex] -4a - [tex] 2a^{2} [/tex]
det(A(a))= [tex]a^{3} [/tex] - [tex]2 a^{2} [/tex] -4a +8
det(A(a))= [tex] a^{2} [/tex] (a-2) -4(a-2)
det(A(a))= (a-2) ([tex] a^{2} [/tex] -4 )
det(A(a))=0
(a-2) ([tex] a^{2} [/tex] -4 ) =0
Caz I: a-2=0
a=2 ∈ R
Caz II: [tex] a^{2} [/tex] -4 =0
[tex] a^{2} [/tex] =4
a= +2 ∈ R sau -2 ∈ R
a ∈ {+2,-2}