Is an Infinite Sum of Rational Numbers Always Rational?

In summary, the logic that an infinite sum of rational numbers that converge must also be rational because two rational numbers added together is still rational is flawed. This can be seen through examples of transcendental numbers such as pi, which can be represented as an infinite sum of rational terms. However, just because something may seem intuitively true in math does not necessarily mean it is logically deduced. The "limit" of an infinite series may be irrational, but the notation of an infinite sum is simply a representation of sum to n where n tends to infinity.
  • #1
captain
164
0
if two rational numbers added together is still rational then wouldn't an infinite sume of rational numbers that converge also be rational and if that is the case then an irrational number is therefore rational which makes no sense though. i don't see where the flaw in this lies because it is logically inconsistent.
 
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  • #2
If you take a partial sum of an infinite series that converge to an irrational number then you would get a rational number. However, the point is that you never stop adding, so it tends to an irrational.
 
  • #3
captain said:
if two rational numbers added together is still rational then wouldn't an infinite sume of rational numbers that converge also be rational
Why would you think that?
 
  • #4
captain said:
if two rational numbers added together is still rational then wouldn't an infinite sume of rational numbers that converge also be rational

no this can easily be seen by looking at .101001000100001... this number is actually transcendental but it’s power series representation has nothing but rational terms i.e.

1/10 + 1/10^3 + 1/10^6 + 1/10^10 + 1/10^15…

Just because something intuitively seems it should be a certain way in math doesn’t mean it is. Math is about what you can deduce logically, not what you feel something should be like.
 
  • #5
Every real number, rational or irrational, is the sum of an infinite number of termiinating decimals. That is, the sum of an infinite set of rational numbers.

For example, [itex]\pi[/itex]= 3+ 0.1+ 0.04+ 0.001+ 0.0005+ 0.00009+ 0.000002+ ...


Why would you think that what is true for a finite sum is necessairly true for an infinite sum?
 
  • #6
the "limit" if the series is irrational, not the actual sum

infinite sum is just a simple notation of writing sum to n where n -> inf.
 
  • #7
thanks for all your help i just wanted to clarify that for myself
 
  • #8
leon1127 said:
the "limit" if the series is irrational, not the actual sum

infinite sum is just a simple notation of writing sum to n where n -> inf.
For an infinite series, the limit of the partial sums is the "actual sum".
 

What are rational numbers?

Rational numbers are numbers that can be expressed as a ratio of two integers, such as 4/5 or -3/4. They can be written in decimal form, but they will either terminate or have a repeating pattern.

How do you find the sum of two rational numbers?

To find the sum of two rational numbers, you must first find a common denominator. Then, add the numerators together and keep the common denominator. For example, if you wanted to find the sum of 1/3 and 2/5, you would first find the common denominator of 15. Then, the sum would be (1/3)(5/5) + (2/5)(3/3) = 5/15 + 6/15 = 11/15.

Can the sum of two rational numbers be irrational?

No, the sum of two rational numbers will always be a rational number. This is because the sum of two fractions can always be expressed as a fraction, as long as a common denominator is found.

What is the smallest possible sum of two rational numbers?

The smallest possible sum of two rational numbers is 0. This can be achieved by adding any rational number to its additive inverse, or negative counterpart. For example, 1/2 + (-1/2) = 0.

Are there any patterns in the sums of rational numbers?

Yes, there are patterns in the sums of rational numbers. For example, if you add a rational number to itself multiple times, the sum will follow a pattern. This is known as a repeating decimal. Additionally, the sum of two rational numbers with the same denominator will always have the same denominator.

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