(b−3a+6)λ_1=1 ; (c−2b+3a−4)λ_1=0 ; d−c+b−a+2+2λ_1=0 ; (b+3a+6)λ_2=1 ; (c+2b+3a+4)λ_2=0 ; d+c+b+a+λ_2=0