Question

Prove that a positive integer $n \geq 2$ is prime if and only if there is no positive integer greater than $1$ and less than or equal to $\sqrt{n}$ that divides $n$

Answer 1

Answer:

Proof given by contradiction.

Step-by-step explanation:

Given that:

$n \geq 2$

To prove:

$n$ is prime if and only if no positive integer > 1 and $\leq$ $\sqrt n$ divides $n$.

Solution:

First of all, let $n$ is a composite number i.e. not a prime number such that:

$n =a\times b$

and $a$ and $b$ are prime and $a$ divides $n$ and $b$ also divides $n$.

Let $\sqrt n = p$

or $n = p\times p$

1. $\underline{a < p}$:

$a$ is prime and is a divisor of $n$.

2. $\underline{a>p}$:

$n = a\times b = p\times p$

We have assumed that $a > p \Rightarrow b$

$b$ is a prime number and is a divisor of $n$.

But we are given that no prime number $\leq \sqrt n$ divides $n$ but we have proved that $b < \sqrt n$ divides $n$.

So, it is a contradiction to our assumption.

Therefore, our assumption is wrong that  $n$ is a composite number.

Hence, proved that $n$ is a prime number.