@jack phil,
jack phil wrote:
This is sort of an offshoot of the "this sentence is false thread". I think the solution to this question might be simpler than the solution to that one, which of course, may have the same solution as whether the chicken or egg came first.
Does a+b=c?
Is it any more correct to say true rather than false?
Is this a nonsense question?
?
I don't really know where to start, so I hope others will.
The exact expression is :
For all a, and for all ,b there exists a unique ,c such that : a+b = c.
And this expression can be proved in the following way :
1) for all ,x (x=x).........................................................................an axiom in equality
2) a+b = a+b ................................................................................from 1 and using Universal Elimination where we put : x = a+b
3) there exists ,c ( a+b =c) .............................................................from 2 and using Existential Introduction
Now for the uniqueness part:
4) a+b =c & a+b = d ..........................................................................Assumption to start a conditional proof.
5) c=d....................................................................From 4 and using the properties of the equality : for all ,x,y,z [ (x=y and y=z) => y=z] and also ,for all x,y ( x=y <=> y=x)
6) [(a+b=c )& (a+b=d) => c=d]....................................from 4 to 5 and using conditional proof
7) for all c,d [ (a+b=c)&(a+b=d) => c=d].......................................from 6 and using Universal Introduction
8)There exists a unique ,c( a+b=c) ..................................................from 3 and 7 and using the definition of uniqueness
9) for all a,b there exists a unique c (a+b=c)......................................by generalization of 8
This innocent little theorem allows mathematicians to do all sorts of substitutions like :
a+b+c+d =x................. x^4+ y^2 +c = w.........-((-x) + abc^2 + 1/x) =y e.t.c
Without that possible substitution ,perhaps, we would not have mathematics at all
Of course if you ask a mathematician what theorem or axiom whatsoever allows him/her to do such a substitution you will get all shorts of nonsenses .