Explications étape par étape :

x² - 3x + 2 = 0

a = 1

b = -3

c = 2

    Δ = (-3)² - 4 × 1 × 2

⇔ Δ = 9 - 8

⇔ Δ = 1

Δ > 0, 2 solutions:

x₁ =  [- (- 3) - √1 ] / ( 2 × 1 ) = ( 3 - 1 ) / 2 = 2/2 = 1

x₂ =  [- (- 3) + √1 ] / ( 2 × 1 ) = ( 3 + 1 ) / 2 = 4/2 = 2

S = { 1 ; 2 }