Prove that $r!=5x+1$ for some $r$ in $\mathbb{N}$ iff $x \in \{0,1\}$

52 Views Asked by At

The statement is true if $r$ is either $0$ or $1$.

At $x=0$ : $r! = 1$ so $r = 0$ or $1$.

At $x=1$: $r! = 6$ so $r = 3$.

For any other value of $x$, $r$ won't belong to $\Bbb N$.

1

There are 1 best solutions below

0
On

If $r$ is an integer $\ge 5$ then $r!$ is a multiple of $10$.

Otherwise you can check it by hand.