adding lists of integers

John E. Franklin · 2011-05-15 14:45:36

Did you know that if you add up a tall
list of integers, and get the correct
answer, then if you count all the integers
up that are odd including the answer if
it is odd, then the number of odd integers
will always be even!! The trick to
this is that the answer must be included.
Here's an example:
18
17
13
10
11
+
____
69
Now count up the odd integers:
17, 13, 11, and 69, that's four, an even number of odds!!
Cool, huh?
I thought this up while talking to my Mom today.

MathsIsFun · 2011-05-15 15:38:47

Odd odds make an odd

John E. Franklin · 2011-05-17 05:15:19

Thank you for the saying!!
But the evenness even works for this list that comes out even.
For example.
13
15
18
+
____
46
Now we can see that their are still and even number of odd numbers: 13 and 15.
Notice the answer is included, but is not odd, so does not add one to the count.
So there will always be an even number of odd numbers if you
include the answer.

bobbym · 2011-05-17 05:23:58

Hi John;

What MIF is saying is if you have an even number of odds in your list the answer will always be even, post#3.

If you an odd number odds in your list than then the answer will be odd. So when you count up up all the odds and then aswer it will be odd + 1 = even. Post #1

John E. Franklin · 2011-05-17 05:38:54

How did you know what MIF was thinking?
I thought he didn't see that there are always an even number if you count the answer,
so I reexplained that.

bobbym · 2011-05-17 05:53:25

Hi John;

Just guessed. Anyway, what I did was prove why it will always work. Thought you were looking for an explanation.

John E. Franklin · 2011-05-18 12:53:07

Oh, I derived it from seeing both sentences in my head:
1. odd + even = odd still, because even never changes it.
2. odd + odd = even because one of the odds changes it.
3. even + even = even because even doesn't change the other one, it skips by 2's.
Then I wondered what would happen if I included the answer, so I
started checking that out, and whammo!! I found one sentence that
satified all the conditions:
1. If you count the answer too, then the number of odds will always be even in a summation!!
Clever, huh? Just one sentence, and no inferences are needed.
Then you can derive backwards too from this to the original statements
by logically thinking it through.

Math Is Fun Forum

#1 2011-05-15 14:45:36

adding lists of integers

#2 2011-05-15 15:38:47

Re: adding lists of integers

#3 2011-05-17 05:15:19

Re: adding lists of integers

#4 2011-05-17 05:23:58

Re: adding lists of integers

#5 2011-05-17 05:38:54

Re: adding lists of integers

#6 2011-05-17 05:53:25

Re: adding lists of integers

#7 2011-05-18 12:53:07

Re: adding lists of integers

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