A new problem, quite simple if we approach it properly.
In a book has appeared rather long operation:
999 to 998 + 997-996 + 995-994 + ... + 5 to 4 + 3 - 2 + 1
words, this is going adding and subtracting numbers in decreasing sequence, from the 999 to 1, the odd numbers are added, the pairs are subtracted. Can you calculate this operation?
The solution, as always, below, so do not expect to see our deductions.
[This illustration matenavegábamos found while looking for images of sums, has been extracted from this page . As you can see, there is a table that presents all sums of two squares less than or equal to 100. What's so funny? Fermat realized that these sums could give composite numbers and prime numbers, but the premiums collected were those of the form 4n +1, that is, that, when divided by 4 gives the remainder 1 (with the exception of 2, that is prime, is the sum of two squares and does not give remainder 1 when divided by 4). In the table are appearing all the cousins \u200b\u200band none of 4n +1 primes 4n +3. This led to enunciate the so-called Fermat Theorem Christmas, a name sometimes given because it is in a letter to Marin Mersenne dated December 25, 1640]
Solution:
may seem silly, we will add to the sum the term zero, which does not alter the result, and wrote:
This way we can match the numbers:
(999-998) + (997-996) + (995-994) + ... + (5 - 4) + (3 - 2) + (1 - 0)
Each pair gives the same result, 1, then we have a large sum of ones. How many around there? Given that the numbers ranging from 999 to 0, there are exactly 1000 numbers, and therefore there are 500 pairs. (The zero we have added to match pairs and counting easier). There are 500 pairs, each one gives 1, then the sum is 500 :
1 + 1 + 1 + ... [500 times] ... + 1 + 1 + 1 = 500.
Note: this problem, slightly modified, has been taken from the textbook 4 º ESO publisher Anaya.