Răspuns:
3
Explicație pas cu pas:
x ∈ N => x + 1 ≠ 0
[tex]\frac{x}{3} = \frac{4}{x + 1} \\ x \cdot (x + 1) = 3 \cdot 4 \\ x \cdot (x + 1) = 3 \cdot (3 + 1) \\ x \in \mathbb{N} \implies \bf x = 3[/tex]
sau:
[tex]\frac{x}{3} = \frac{4}{x + 1} \\ {x}^{2} + x = 12 \iff {x}^{2} + x - 12 = 0 \\ (x + 4)(x - 3) = 0 \\ x + 4 = 0 \implies x = - 4 \not \in \mathbb{N} \\ x - 3 = 0 \implies \bf x = 3 \in \mathbb{N}[/tex]
