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May 26, 2015

Posted by **Mia** on Thursday, September 24, 2009 at 4:50pm.

p=a^2+b^2=c^2+2 e^2=f^2+3 g^2, and show how it's done with each one. The numbers you're squaring must be integers; show your thinking. thanksi dont get it at all

- Math(grade8) -
**Mia**, Thursday, September 24, 2009 at 5:03pmwhy does noone answer my question and everyone elses

- Math(grade8) -
**MathMate**, Thursday, September 24, 2009 at 5:43pmI'll bite. I am interested in the question, but I need to know more about the "rules".

It sounds like a number theory problem, but you marked it as grade 8. Can you tell me a little more about what you have been learning about with prime numbers?

Do a and b have to be distinct?

For example, does

2=1²+1² count?

Do c,d have to be distinct?

Do e,f have to be distinct?

Did your teacher give an example? If yes, can you post the example?

- Math(grade8) -
**Mia**, Thursday, September 24, 2009 at 6:19pmEx this is what it says

Some primes can be expressed as the sum of two squares, as in 13=2^2 +3^2; some can't:7=/= a^2+b^2. Other primes can be c^2+2 e^2, like 11 =3^2+2

* 1^2; still others are f^2 + 3 g^2, like 31 =2^2 +3 *3^2 thats what the sheet says he didnt really explain he just said do it and me and my friends are all confused and yes its grade 8. we only learned like what numbers are prime/composite but were doing like lcm &gcf so this question was kinda random

- Math(grade8) -
**MathMate**, Thursday, September 24, 2009 at 6:46pmI believe this exercise is to encourage organization of work.

Indeed, there are primes under 100 that satisfy these properties, so your teacher does not make you work for no reason. To answer my earlier question, some integers are re-used in composing the same prime.

The key is to search systematically. It may look like a lot of work, but it is not that difficult.

Count the number of primes between 1 and 100 (1 is not a prime). Make a table of four columns and 25 rows. The first column contains the primes, 2,3,5,7,11...

The second column is the sum a²+b&sups;, the third is the sum c²+2d&sups;, and the fourth is the sum e²+3f&sups;.

Start from 1 and 1:

- calculate 1²+1²=2. It is not a prime, do nothing.

- calculate 1²+2*1²=3. It is a prime, so in the square corresponding to the prime 3 and under (third) column for c & d, mark 1²+2*1².

- calculate 1²+3*1²=4. It is not a prime, do nothing.

Repeat for (1,2), (1,3)....until (1,9). 10 is not a candidate, since 10²=100, the upper limit.

Next, do (2,1), (2,2), ...

then (3,1), (3,2)....

until (9,9).

If a row contains three checks, this is your answer. You should find two primes.

If your teacher shows you a mathematical formula to find the answer, I would like to hear about it.

- Math(grade8) -
**Mia**, Thursday, September 24, 2009 at 6:47pmwoah dude how come u know so much

- Math(grade8) -
**Mia**, Thursday, September 24, 2009 at 6:48pmand thank you so so so so so much for your help :) ill let you know

- Math(grade8) - corr. -
**MathMate**, Thursday, September 24, 2009 at 6:49pmCorrection:

The second column is the sum a²+b², the third is the sum c²+2d², and the fourth is the sum e²+3f².

Another advantage of the exercise is that by the end of your work, you will be very familiar with all the primes under one hundred.

- Math(grade8) -
**MathMate**, Thursday, September 24, 2009 at 6:51pmYou're welcome!