[tex]C_x^3+C_x^2=15(x-1)\\
\frac{x!}{(x-3)!3!}+\frac{x!}{(x-2)!2!}=15(x-1)\\
\frac{(x-2)(x-1)x}{6}+\frac{(x-1)x}{3}=15(x-1)|\cdot 6\\
(x-2)(x-1)x+2(x-1)x=90(x-1)\\
(x-2)(x-1)x-88(x-1)=0\\
(x-1)(x^2-2x-88)=0\\
x=1 sau\quad x^2-2x-88=0\\
Deci x=1
[/tex] pt ca rad ec de grad 2 nu sunt naturale pt ca delta=356, deci nu e patrat perfect, deci solutiile vor contine radicali, iar in cazul combinarilor nu putem folosi decat nr naturale.
