Divisibility Tests and their application on finding Prime numbers.

In this video we will learn divisibility tests from 2 to 13 and will learn their application on solving an important class of questions which comes in a lot of examinations.

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[duonghoangminh]

Tbh, you only need to test for 2, 3, 5, 7, 11, 13 because if they're divisible by 4, 6, 8, 10, 12 then they're divisible by 2 and if they're divisible by 9 then they're also divisible by 3

[suryajustin]

wow ! this is clear, concise, and complete, always you make me want to learn more !

[sr]

On the 157. You shouldn’t need to check for 13 as 13 squared is 169. If any number below 169 is divisible by 13 then you will have already found a divisor! Another interesting for 13. Multiply it by 7 and then 11 it will be a multiple of 1001. Example 273 x 7 x 13 gives 21021

[ms_majhi]

Hey. U only need to check divisibility for 1 to square root of that number. For example: u want to know if 101 is prime. Square root of 101 is 10.1(suppose), u only need to check divisibility for 1 to 10 . And also u don't need to check for even number bcoz if the number is divisible by even number then it will also be divisible by 2 . So it can't be prime(except if the number itself is 2).
So for 101 we only need to check for 2, 3, 5, 7, 9.

The reason is simple.
Suppose the number is 100.
Square root of 100 is 10.
So. 10x10 = 100.
(Less than 10) x (Less than 10) = Less than 100)
(Greater than 10) x (Greater than 10) = Greater than 100

So two numbers will only make 100 when one is less than 10 and the other is greater than 10.

So if there is an integer less than 10 which divides 100 perfectly then there must be an integer greater than 10 which also divides 100 and together they make 100 when multiplied by each other.

And if there is no number less than 10 which divides 100. Then there can't be a number greater than 10 which does the same.

I hope this is helpful

[Adityaaa]

I wish you uploaded it yesterday, i had my maths exam today....

[anxiousoptimism]

This was very informative. Thank you for making this video.

[vishalmishra]

*You can invent your own divisibility tests for the next prime number or use algebra to verify the existing ones* . Here is how -
Assume A is a positive integer and b is a single digit. So, Ab is mathematically (10A+b). To invent your divisibility test of 7 or 13 or 17 -
(10A + b) = 10 (A - 2b) + 21b. Since 21 is a multiple of 7, you just need to check if A - 2b is a multiple of 7 (10 is ignored because it's not a multiple of 7 and has no common factors).
(10A + b) = 10 (A + 4b) - 39b. Since 39 is a multiple of 13, you just need to check if A + 4b is a multiple of 13 (10 is again ignored since it's not a multiple of 13 and has no common factors).
(10A + b) = 10 (A - 5b) + 51b. Since 51 is a multiple of 17, you just need to check if A - 5b is a multiple of 17 (10 is again ignored since it's not a multiple of 17 and has no common factors).
So, divisibility test of 17 is the same as 7 except multiply the last digit by 5 instead of 2 and check if the remaining (smaller by 1 digit) number is a multiple of 17. Repeat as needed if its not small enough. *Thank you for sharing your videos. I did not know the divisibility test of 7 but in an attempt to verify it's correctness, I was able to invent my own test for 13 which later matched as your video continued into the next prime. I added divisibility test of 17 to prove that this method of inventing new divisibility tests works for any next prime number* .

[mrTanjiro_kamado]

Your class make me to get more knowledge (maths)🤩

[technotactics]

I HAVE AN EXAM ON THIS TOMMOROW THANK YOU SOO MUCH

[jesseelechi]

Division by 11 is easier if you subtract the last digit by the rest

Example 121 = 12-1 =11

[yanyan]

How to cube 2 digit number pls, make a video about it.

[srivatsavan]

Can you please upload a vedio on chika's rule

[choti]

How I do is test for 2 by add that number and test for 2. Similarly add other number individually and test for 3, 5 and 7 . Finally 13. Much faster

[HeroProMan]

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Brother thank you very much and thanks a lot for uploading video I was really sad because you were not uploading the video but today I am very happy and I am very happy because today is my luckyest day to watch you.

[chinoutugirl]

Checking if 147 is divisible by 7:
7×2=14
14-14
0 ans

[RAHULKUMAR-juqs]

Prime no generally in the form of 6n+1 or 6n-1 for >3

[WasteChennel]

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Anyway I don't care because I hope that if you are both are not brother but in which manner and you both are talking I hope that you were always be a friend or a brother not in real life but in online you are always be brother I wish to God and pray to God to make your friendship or family nice always together

[EustacheAlemba]

I will never struggle with finding prime numbers any

[UnknownParadox-ok]

5:37
Why u added 5+2 while
5 is not a even number..?

[alokpink]

Please do you upload English speaking classes please 🥺🥺🥺🙏🙏🙏