Monday, December 24, 2012

HH #391, is That a Primary Number?

Esser Agaroth is the host of Havel Havelim #391First of all, to answer my question.  391* isn't on the list I found of primary numbers.

Now for this very special Havel Havelim that Ya'aqov has compiled.  There are lots of posts from various Jewish bloggers. 

Havel Havelim is a weekly blog carnival on Jewish and Israeli topics.  Bloggers send their links in to the week's host, and the hosts can (and should) add other blog posts he/she sees on the net.  We communicate via our facebook page, volunteering to host each week.  Participating in HH is a good way to promote one's blog and get to know other blogs.  Some of my most read posts have been the ones that are Havel Havelim and the Kosher Cooking CarnivalA look at the list on "Top 10" on my sidebar will show you that.

Next week's host will be Beneath the Wings, who has requested that the links be sent via fb message,  details on our facebook page.

*I have no time today to calculate, so can anyone please show how to divide 391, please, thanks?


Leah, Maaleh Adumim said...

391 is 17 * 23. to find out if a number is prime (or to factor a number), divide by prime numbers up to the square root of the number to see if any of them go in evenly. so:

391 isn't divisible by 2. (we can see that easily because it's not even - i.e. doesn't end in 2, 4 6, 8, or 0)

it's not divisible by 3 (we can see that easily because the sum of its digits is not divisible by 3)

it's not divisible by 5 (we can see that it doesn't end in 5 or 0)

it's not divisible by 7, 11 or 13 (try it and see)

I am skipping all the composite numbers along the way, because obviously if it's not divisible by 2 or 3 then it won't be divisible by 4 or 6 either.

so now we are up to 17 - bingo! and we get the other factor too: 23. 17 and 23 are both prime, so no need for any further factoring. (e.g. if we would say that 24 is divisible by 6 and 4, we can break down each of those into additional prime factors and get 3, 2, 2, 2 as the factors of 24.)

Batya said...

Leah, thanks. I was thinking that there could be a "7" there, because 3x7=21. You are a great Math teacher!

Leah, Maaleh Adumim said...

thanks for your kind words. but yes, this is one type of problem that I often help my students with.

Batya said...

no surprise