Réponse:
1)
A = 3²-2×3×x + x² - (3×5 + 3×x - 5×x - x×x) + 5×9 - 5 ×x²
A = 9 - 6x + x² - 15 - 3x + 5x + x² + 45 - 5x²
A = -3x² - 9x + 39
2)
A = (3-x)(3-x) - (3-x)(5+x) +5(3-x)(3+x)
A = (3-x)[(3-x) - (5+x) + 5(3+x)]
A = (3-x)( 3 - x - 5 - x + 15 + 5x )
A = (3-x)(3x+13)
3)
A =0 <=>
(3-x)(3x+13) = 0 <=>
(3-x)=0 ou (3x+13)=0 <=>
x = 3 ou 3x = -13 <=>
x=3 ou x = -13/3
S ={-13/3; 3}
A = 39 <=>
-3x²-9x+39=39 <=>
-3x²-9x = 0 <=>
-3x(x+3)=0 <=>
-3x = 0 ou x+3 =0
x = 0 ou x=-3
S ={-3; 0}
