Quick tip to see if a number is prime

A simple test to see if a number is prime or composite. All you do is find the square root of the number. Next, find all the prime numbers of this number. If none of these prime numbers are a factor of the number then the number is prime.

[matiaspereira]

If you want to find out if a big number is prime or composite you have to find out its square root and divide it by all prime numbers smaller than its square root. If the number is divisable by any of the prime numbers then the number is composite but if the number isn't divisable by any of the prime numbers then the number is a prime

[nietocurcio]

Thank you, it helps a lot. I was solving a programming problem, and I think that we should check all prime numbers <= square(n), not only prime numbers < square(n). 361 is not prime, its square root is 19, 19 is a prime number that divides 361 into 19 parts and makes it not prime. It's an edge case that the result of the square root is a prime number as well.

[thomaservin]

Just memorize several dozen primes. It is slightly worth knowing.
Note also: all primes are odd numbers, except 2. All even numbers are not primes. No primes end in zero, nor 5. This bit of trivia can help you to identify non-primes, at least. So all primes end in 1, 3. 7, or 9 (the reverse is not true though

[jtteope]

High school: Thank you this helped a lot
Grade 3: *CONFUSED*

[rubenyanez]

for finding the square root of the number, wouldn't it be better to say the first step is to find the closest perfect square above that number?

[dvsbtrwneryynfdgsbgsgb]

How can we determine if 1009 is a prime number? According to this method, root of 1009 is about 31 so we need to divide the prime numbers from 0 to 31? It is really hard to calculate.

[sch]

I watched 5 videos this method is the only method that is truly quick thanks!

[robertodelaprieta]

The statement "For any composite number (X) it's prime factors must be less than the square root of the number" in 0:25 is NOT TRUE. Take, for instance,

14 = 7x2
15 = 5X3

It is however true that "every composite number has a prime factor less than or equal to its square root".

[metayoutubeacademy]

this video recomended by google after searching "fastest way to know if the number is prime"... it helped me alot thats why i come to comment here to say thak you.... this video deserve millions and trillions of likes and views

[Deliver-Fool]

For the last sentence in the summary, is it not true that 1) If initial number is not divisible by any primes less than OR EQUAL TO the square root of the inital number, then it's prime?

For example, the square root of 49 is 7 but if it's only LESS THAN square root, then 49 is prime BUT...

if it's less then OR EQUAL to, 49 is not prime.

49 is not a prime number because it has more than 2 factors (1, 7 49)

What does everyone think? 😎

[rossanaleblanc]

Good method but I will inform you, or maybe you forgot, there is a very easy test for finding out if a number is divisible by three: add up the digits and they must be divisible by 3. So 9+7=16 and 2+9=11 tell me neither 97 nor 29 are divisible by 3.

[vschannel]

Thanks. Your trick makes sense. The other 6j-1 and 6n+1 is not that efficient. Thanks a lot

[sabikikasuko]

I don't think a methodology for finding prime numbers that requires knowing which numbers are prime is very useful...

[HarshYadav-cjyu]

thanks for such a great lesson.
I really appreciate that

[llg]

what about a number like 361? is there a way without using my calculator to determine that it has 1 higher-than-normal prime factor, and is not a prime number? Because I normally would not keep trying all the way up to 19!

[rubenyanez]

first tip is useless, any number that has a perfect square we can automatically deduce that that is not prime

[dd-hidm]

How to know if a large no. Is prime no for example 1237

[tiktikjeihani]

Do i miss something or it doesn't work for 9. Square root of 9 is 3. Prime less than 3 is 2. 9 is not divisible by 2. So 9 is prime?

[JoshuaLi-pe]

question: what about for really, really big numbers, like 2958723495732098573401?

[itz_jenjelly]

There's an super Easy way to do this

In which you have to Remember that the number which only have a 2 factors 1 and the number itself is called prime number so if we will take the example of 167 it is a prime number as it only has 2 factors which are 1 and the number itself 😊.

And If you want To find out that the number is composed or not that you have to just remember that composite numbers are the numbers that have more than 2 factorso they are called composite numbers we can take the example of 161 because it has more than 2 factors