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*I have no time today to calculate, so can anyone please show how to divide 391, please, thanks?
4 comments:
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.)
Leah, thanks. I was thinking that there could be a "7" there, because 3x7=21. You are a great Math teacher!
thanks for your kind words. but yes, this is one type of problem that I often help my students with.
no surprise
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