Bonjour,
a) 2 + 3/x = 0
⇔ (2x + 3)/x = 0
⇒ 2x + 3 = 0
⇔ x = -3/2
b) 1/x + 1/2 = 3
⇔ 1/x + 1/2 - 6/2 = 0
⇔ 1/x - 5/2 = 0
⇔ 2/2x - 5x/2x = 0
⇔ (2 - 5x)/2x = 0
⇒ 2 - 5x = 0
⇔ x = 2/5
c) x + 42/x = 0
⇔ (x² + 42)/x = 0
⇒ x² + 42 = 0
⇔ x² = -42 impossible donc pas de solution