Question

Помогите решить уравнения по комбинаторике

x/A³ₓ = 1/12 и 20*x = A²ₓ

Answer 1

$\frac{x}{A^3_x} = \frac{1}{12} \Rightarrow 12x = A^3_x \Rightarrow \frac{x!}{(x - 3)!} = 12x \Rightarrow x(x - 1)(x - 2) = 12x \Rightarrow x^3 - 3x^2 - 10x = 0 \Rightarrow x(x + 3)(x - 5) = 0 \Rightarrow x = 5$

$20x = A^2_x\Rightarrow 20x = \frac{x!}{(x - 2)!} \Rightarrow 20x = x(x - 1) \Rightarrow x^2 - 21x = 0 \Rightarrow x(x - 21) = 0 \Rightarrow x = 21$