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Proof: sum & product of two rationals is rational

Sal proves that the sum, or the product, of any two rational numbers will always be a rational number. Created by Sal Khan.

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  • starky seed style avatar for user theshadow6606
    look, I'm genuinely confused about this rational thing.
    he said that if an int is divided by an int then its rational right?
    but 2/7 = 0.285714 recurring
    I'm pretty certain that's not rational.
    my teacher couldn't explain this to me

    edit: would love if someone answered this quickly
    (10 votes)
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    • mr pink green style avatar for user David Severin
      So what makes you think it is not rational? If a decimal is repeating, it should be rational because some people such as myself can relatively easily find the two whole numbers to create a fraction. All truncating and repeating decimals are rational because they meet the definition of being a ratio of two integers or whole numbers.
      An irrational number has a decimal that NEVER repeats. So if it repeats, then it does not meet the qualification of NEVER.
      (15 votes)
  • male robot donald style avatar for user yash jaiswal
    can anyone of you can tell if this is true or not?
    1+2+3+4+5.....to infinity= -1/12?

    want to know just upvote for this question
    (5 votes)
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    • female robot ada style avatar for user ✨🌺🍧
      No, because this is not a series whose terms are approaching zero. The terms are increasing by 1, and this is an arithmetic series. So, no, you can't calculate the sum of this series. It just approaches infinity.

      In fancy terms, we would say that it 'diverges', that is, it evaluates to some really big number we can't bound.
      (5 votes)
  • piceratops tree style avatar for user arhantcolluru2010
    but when we multiply 1/7 and 22/1 we get 22/7 which is irrational and same with addition when we add 1/7 and 21/7, we get 22/7
    (2 votes)
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    • stelly blue style avatar for user Kim Seidel
      22/7 is a rational number (a fraction created with 2 integers). It creates a repeating decimal. It just has a long repeat pattern of 6 digits.

      If you think it is irrational because you think it is equal to Pi, that is a common point of confusion. 22/7 is just an approximation of Pi. Pi is irrational (a non-terminating and non-repeating decimal). Compare their decimal values. By the 3rd decimal digits, their values are different.

      22/7 = 3.142857142857142857...
      Pi = 3.14159265358979323846264338...

      Hope this helps.
      (7 votes)
  • duskpin seedling style avatar for user Dre Hill
    How do you find out which is which? (Irrational & Rational)
    (4 votes)
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    • purple pi pink style avatar for user YDW Mv!k0m
      Rational Numbers: The real numbers which can be represented in the form of the ratio of two integers, say P/Q, where Q is not equal to zero are called rational numbers. Irrational Numbers: The real numbers which cannot be expressed in the form of the ratio of two integers are called irrational num
      (2 votes)
  • spunky sam blue style avatar for user andersonwyatt199
    How do we know the sum of two integers is an integer?
    (3 votes)
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  • sneak peak purple style avatar for user Vector Inc.
    At , when Sal added bn + bn, how come he didn't write it as 2bn? Since he added both of them together and they are the same quantity, shouldn't he have written it as 2bn?
    (2 votes)
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  • duskpin ultimate style avatar for user Pierre Dob
    Isn't it the case that this proof is false because sal's reasoning ends by assuming the proof is true (we can't know if "am" is a rational number because that's what we're trying to prove).
    (2 votes)
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    • female robot grace style avatar for user IOn
      Rational numbers are defined as the numbers that can be written as the ratio of two integers.

      We take two rational numbers a/b and m/n
      which means that a, b, m and n are integers
      according to the definition of rational numbers.

      We want to know if the product of two rational numbers is also a rational number, so we multiply a/b by m/n

      which equals to (a*m)/(b*n)

      a*m and b*n are both integers, because multiplying an integer by an integer gives us an integer.

      So (a*m)/(b*n) is also a ratio of two integers,
      which makes it a rational number, because that's how rational numbers are defined.
      (3 votes)
  • purple pi purple style avatar for user The first integral proponent
    What about irrational divide irrational? rational divide irrational? irrational divide rational? irrational to a rational number's power? irrational to irrational? rational to irrational?
    (3 votes)
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  • blobby green style avatar for user aviralp
    is zero a rational number or a irrational number
    (1 vote)
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  • spunky sam green style avatar for user m.prosser2022
    So If you do X + 34 + 9/33 x 44 = y =555 what would add 33+9/33x44
    (3 votes)
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Video transcript

What I want to do in this video is think about whether the product or sums of rational numbers are definitely going to be rational. So let's just first think about the product of rational numbers. So if I have one rational number and-- actually, let me instead of writing out the word rational, let me just represent it as a ratio of two integers. So I have one rational number right over there. I can represent it as a/b. And I'm going to multiply it times another rational number, and I can represent that as a ratio of two integers, m and n. And so what is this product going to be? Well, the numerator, I'm going to have am. I'm going to have a times m. And in the denominator, I'm going to have b times n. Well a is an integer, m is an integer. So you have an integer in the numerator. And b is an integer and n is an integer. So you have an integer in the denominator. So now the product is a ratio of two integers right over here, so the product is also rational. So this thing is also rational. So if you give me the product of any two rational numbers, you're going to end up with a rational number. Let's see if the same thing is true for the sum of two rational numbers. So let's say my first rational number is a/b, or can be represented as a/b, and my second rational number can be represented as m/n. Well, how would I add these two? Well, I can find a common denominator, and the easiest one is b times n. So let me multiply this fraction. We multiply this one times n in the numerator and n in the denominator. And let me multiply this one times b in the numerator and b in the denominator. Now we've written them so they have a common denominator of bn. And so this is going to be equal to an plus bm, all of that over b times n. So b times n, we've just talked about. This is definitely going to be an integer right over here. And then what do we have up here? Well, we have a times n, which is an integer. b times m is another integer. The sum of two integers is going to be an integer. So you have an integer over in an integer. You have the ratio of two integers. So the sum of two rational numbers is going to give you another. So this one right over here was rational, and this one is right over here is rational. So you take the product of two rational numbers, you get a rational number. You take the sum of two rational numbers, you get a rational number.