Ask HN:properties of real numbers

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	Ask HN:properties of real numbers
	2 points by pencil 5729 days ago
	Hello, The addition property of real numbers says if a=b,c=d then a+c=b+d.can someone tell me why is it true? is there an algebraic proof for this or should we accept this as being true based on inductive reasoning?i really tried doing a google search for a proof but couldn't find any. (i know i had asked a similer question a couple of days ago but i feel i might get a much more reasonable answer to this one..may be the way i had put accross the question wasn't sensible)

7 comments

gdl 5729 days ago

Informal reasoning: in a=b, the 'a' and 'b' are merely placeholders that act as pointers to the same underlying value, so they can be used interchangeably. It's no different than referring to "seven" and "the integer directly following six" - I could use either reference any place I could use the other with no effect on the overall statement. Ditto for c=d. All that is happening with a+c=b+d is that two references to values are being changed to different references with the exact same underlying values, so the result is necessarily the same.

See also http://en.wikipedia.org/wiki/Peano_axioms if you prefer the math and logic jargon. High school geometry taught me to dislike dealing with formal proofs, but I think that should be about the right area to look.

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pencil 5729 days ago

i know even i used to hate formal proofs!!!i assumed this might have a formal proof by not knowing that this property is based on informal reasoning. (but i personally like formal proofs!!!!!)

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patio11 5729 days ago

There are a bunch of ways to prove this. Let's start with addition, subtraction, equality being commutative, that a + 0 = a, and that a - a = 0

Suppose that d != a - b + c. Since a = b, a - b = 0. This implies d != c. This is a contradiction, so our supposition is inaccurate. d = a - b + c

Now, suppose a + c != b + d. Plug in what we just learned. a + c != b + a - b + c. The b's cancel, leaving another contradiction. Thus, supposition inaccurate, so a + c = b + d. QED

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sleepdev 5729 days ago

Tangentially related question: how are real numbers formally defined?

I remember that integers are usually defined in terms of successors: Succ 1 = 2. But this doesn't help for real numbers because they can't really be enumerated?

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pencil 5729 days ago

well..this gives the definition of real numbers http://en.wikipedia.org/wiki/Real_numbers..but doesn't mention even a single bit about the truthfullness of the properties of realnumbers.(to be honest i'am not in a position to come out with a rational explanation!!!!!)

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mfukar 5729 days ago

a = a (equality is reflexive under Peano arithmetic)

=> a + c = a + c (addition is commutative under Peano arithmetic)

=> a + c = b + c (a == b)

=> a + c = b + d (c == d)

qed

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KoZeN 5729 days ago

if a=b,c=d then a+c=b+d

Also, d= a-b+c

Probably doesn't help but thats about the limit of my capabilities!