Question

Помогите пж очень срочно

Answer 1

$C_{x}^{3} =2C_{x}^{2} \\\\\frac{x!}{3!*(x-3)!}=2*\frac{x!}{2!*(x-2)!}\\\\\frac{(x-3)!*(x-2)*(x-1)*x}{3!*(x-3)!}=\frac{(x-2)!*(x-1)*x}{(x-2)!}\\\\\frac{x(x-1)(x-2)}{1*2*3}-x(x-1)=0\\\\x(x-1)(x-2)-6x(x-1)=0\\\\x(x-1)(x-2-6)=0\\\\x(x-1)(x-8)=0\\\\x\neq0;x\neq1\\\\x-8=0\\\\x=8\\\\Otvet:\boxed{8}$