# A math expression I can't think a solution for

Discussion in 'Community Discussion' started by Mammoth, Jul 28, 2007.

1. ### Mammoth macrumors 6502a

Joined:
Nov 29, 2005
Location:
#1

Okay, for this equation/expression to be solved at all, we must adhere to 2 rules/assumptions:

1. Infinity (∞) is never ending and does not stop under any circumstances
2. Subtracting anything from itself always equals zero

Now for the equation that I can't solve:
∞-∞=x
Solve for x.

Now it may seem with rule 2 that the answer is zero. But rule 1 says that ∞ is never ending, no matter what is taken away. Even though you are taking away infinity from itself, it is still infinity because it never ends.

Now that you see what I am thinking about this, you can see what I am thinking that could be an explanation:

1. ∞ is not a number thus rule 2 does not apply
2. Taking ∞ away from ∞ equals zero because rule 2 applies no matter how high the number

Sorry if this is confusing, I'm cobbling this together as I type, hence the potential confusion.

...So can it be explained?

2. ### sammich macrumors 601

Joined:
Sep 26, 2006
Location:
Sarcasmville.
#2
x = ∞

Why? my guess:

We can say: ∞ is "some unfathomably large number" and '∞' does not equal '∞'.

∞(1) = k * ∞(2) where 'k' is any number.

So: ∞(1) - ∞(2) does not necessarily equal 0.

Joined:
Dec 2, 2006
Location:
the OC

Joined:
Dec 19, 2002
Location:
NYC
5. ### Mammoth thread starter macrumors 6502a

Joined:
Nov 29, 2005
Location:
#5
Good article. And here's the answer:

So I guess that means..

∞-∞=∞+1

Which I guess couldn't make sense because (Again a rule that isn't official) any positive number that is subtracted from a greater positive number always yields an answer lesser than the greater number. Example:
a-b=c
a>b, a>c

6. ### FredAkbar macrumors 6502a

Joined:
Jan 18, 2003
Location:
Santa Barbara, CA
#6
From what I can see, the problem (no pun intended) is that you (or they, whoever wrote the problem, I suppose) are using infinity in an equation as if it were a number, while really it's just a concept, usually meaning "infinitely large." So by "infinity - infinity," perhaps what is really meant is "a really big number minus itself."

In math speak, it would be like, the limit (as n approaches infinity) of the expression (n - n). This limit is 0. As long as the two infinity symbols both correspond to the same number (it can't actually correspond to infinity itself, as this isn't a number at all), then the answer is always 0.

7. ### sikkinixx macrumors 68020

Joined:
Jul 10, 2005
Location:
Rocketing through the sky!
#7

File size:
21.5 KB
Views:
25
8. ### Bobdude161 macrumors 65816

Joined:
Mar 12, 2006
Location:
N'Albany, Indiana
#8
oooo that is gruesome. i like.

9. ### siurpeeman macrumors 603

Joined:
Dec 2, 2006
Location:
the OC
#9
what's up with these rules? who says we have to deal with positive numbers? and who says a has to be greater than b?
a - b = c
c can most certainly be greater than a. and for the record, ∞ + 1 is still ∞.

10. ### After G macrumors 68000

Joined:
Aug 27, 2003
Location:
California
#10

You all are just reading it the wrong way.

Then the equation should read as follows:

8
|
8
||
X

The answer should be easily computed afterwards.

(the above was just my cracked up sense of humor; the answer really can be anything, because infinity can be different sizes.)

11. ### malenfant macrumors member

Joined:
Dec 31, 2006
#11
∞ - ∞ = ∞

an easy way to demonstrate this:

say you have a hotel that contains an infinite number of hotel rooms, and each room has a number. each room has one person in it. the front desk calls all of the rooms that have an even room number, and asks everyone to leave, so now only the rooms with odd numbers have people in them. an infinite number of people have left the hotel (since there are an infinite number of rooms with even numbers), but there are still an infinite number of people in the hotel (since there are an infinite number of rooms with odd numbers). qed: ∞ - ∞ = ∞.

12. ### cube macrumors G5

Joined:
May 10, 2004
#12
Infinity is not a number, and some infinite sets are bigger than others (aleph number, there's an infinite (countable) number of sizes).