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Algebra 1

Course: Algebra 1>Unit 15

Lesson 3: Proofs concerning irrational numbers

Proof: √2 is irrational

Sal proves that the square root of 2 is an irrational number, i.e. it cannot be given as the ratio of two integers. Created by Sal Khan.

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• How can a fact that the assumed numbers are reducible, proves that root 2 is irrational. I didn't get the logic.
• Games have rules and math has rules, so in that sense, math is like a game. If you follow the rules of math, you win. What do you win? You win a truth, which up till now, you have been calling “a correct answer” – but there is more. . .

You already know many math rules, like keeping an equation balanced, or a negative number multiplied by another negative number results in a positive number etc. etc. . . . . There are lots of these kinds of procedural rules for playing with numbers.

But there are other types of rules that will become more and more important as you get further into math and they have to do with the properties of numbers and consequences of these properties. The most amazing thing about these rules is that they, combined with other rules can lead you to a truth than no one has discovered yet, in other words, this rule based game of exploration can take you where no one has gone before – and it is that aspect which keeps us pushing the boundaries of mathematics. Many new discoveries are beautiful (fractal geometry), or useful (chaos theory), or weird (quantum mechanics) and some are even a bit disturbing (the n-body problem).

Now, the path that leads to a truth in mathematics is called a proof. Guess what? You have been doing proofs all this time, right since you first started to add numbers up until now. For example, if you have a problem like 7x - 10 = 5x + 6, you can prove, using the rules of the game of math, that x can only be equal to 8 in order that 7x - 10 = 5x + 6 becomes a true statement. This is an example of a proof using just the procedural rules. If at each step of the way, you obey the rules, you prove (arrive at the truth) that x = 8.

The proof you are asking about in this video is a proof that uses some properties of numbers and some concepts and their consequences. This takes a bit more considered thinking. The proof that √2 is irrational is the most common introduction to this type of thinking.

So, here we go . . . . .

First, would you agree that any rational number whose numerator and denominator are not co-prime, can be reduced to a co-prime form? (if you don’t agree, look into it, because it is true). Co-prime is just a fancy pants way of saying that the greatest common divisor between two numbers is 1. Lots of times you will hear it said as “reduced form”. So if you have 4/6, you can take out the 2 which is a common term between 4 and 6, to get 2/3 and then the only factor that 2 and 3 have in common is 1, right? This process is true for any rational number, that is, a rational number that is not already in co-prime form can be reduced to co-prime form. So, for example 6/9 = 4/6 = 2/3, the ratio is the same, the result is the same even though the numbers are different. This little detail becomes important in the proof . . . .

A common method of proof is called “proof by contradiction” or formally “reductio ad absurdum” (reduced to absurdity). How this type of proof works is: suppose we want to prove that something is true, let’s call that something S. If we start the proof by assuming that S is false, and then through a series of mathematically sound arguments, we can show that we get a nonsense or contradictory result, well then, that means that the assumption we made that S was false can’t be correct, so S must be true.

In this proof we want to show that √2 is irrational so we assume the opposite, that it is rational, which means we can write √2 = a/b. Now we know from the discussion above that any rational number that is not in co-prime form can be reduced to co-prime form, right? So for the sake of argument, let’s assume that we have done any needed reduction and now a and b are reduced (co-prime), meaning that the only common divisor between the numerator and denominator is 1. Now we square both sides of √2 = a/b to get 2=a²/b² and from there it is a short journey to show that since 2=a²/b², it means that both a and b must be even numbers which means they have a 2 in common. Oops! We said that they only had a 1 in common.

Here is the subtle part:

Even though it is true that having the number 2 in common is in contradiction to the statement that a and b are in reduced form (co-prime), this contradiction is not the big deal. The big deal is that we got to this stage by assuming that √2 was rational in the first place! By assuming that √2 is rational, we were led, by ever so correct logic, to this contradiction. So, it was the assumption that √2 was a rational number that got us into trouble, so that assumption must be incorrect, which means that √2 must be irrational.

https://nrich.maths.org/4717

as an aside, you might want to look up the definition of mathematical soundness:
https://en.wikipedia.org/wiki/Soundness
• has anybody looking up the digits of sqrt 2 noticed that there is a bit of pi in the sequence of its digits? first you have 1.41 as the first 3 digits of sqrt 2. They are also the first 3 decimal places in pi. Later on in the sequence you have 3141 which are the first 4 digits of pi. If it continues like this do you think that very deep in the sequence of the digits of sqrt 2 that there might actually be the number pi? if so why isn't pi constructable like sqrt 2 is? and why isn't it algebraic like sqrt 3?

constructable number: number you can construct with a straightedge and compass like the Greeks did

algebraic number: number that is a root of a non-zero polynomial in one variable with rational coefficients (or equivalently—by clearing denominators—with integer coefficients)

transcendental: neither algebraic or constructable
• Because the digits of √2 do not repeat forever, it is likely that any desired sequence of digits will eventually occur somewhere in √2. You can likely find the first X digits of pi somewhere in √2. You can likely find your phone number, or any long strings of consecutive zeros. I only say likely though, because it hasn't been proven that irrational numbers behave this way. It goes way beyond the scope of an answer here, but see these articles for some relevant theories: http://www.lbl.gov/Science-Articles/Archive/pi-random.html http://mathworld.wolfram.com/BBPFormula.html
• I get lost at when Sal says 2b^2 = 4k^2. It seems to come out of nowhere. Why does the "2k^2" change to "4k^2?"
• He shows that a is even. So therefore a = 2k, and a^2 = (2k)^2. So:

(2k)^2 = (2k)*(2k) = 2*k*2*k = 2*2*k*k = 4(k^2) = 4k^2
• Can't I write √2 as √2/1?
• Yes but the rule to determine rational numbers is that it can be expressed as a fraction of two integers, and √2 is definitely not an integer.
• How would we prove that the cube root of 4 is irrational using this method?
• Good question! We can do a similar proof.

Assume that cuberoot(4) is rational. Then we can write cuberoot(4) in the form a/b, where a and b are integers, b is nonzero, and a/b is in reduced form (irreducible).

Cubing both sides gives 4=a^3/b^3 which would give 4b^3=a^3. Since 4b^3 is clearly even, a^3 is even. Since the cube of an odd number is odd, a must be even.

So we can write a=2k for some integer k. Therefore, 4b^3=(2k)^3=8k^3 which gives b^3=2k^3. Since clearly 2k^3 is even, b^3 is even, which then implies b is even (again since the cube of an odd number is odd).

So a and b are even, contradicting the irreducibility of a/b. This proves that cuberoot(4) is irrational.
• Not clear first we assumed a/b is irreducible then, Sal said a/b is even why?
• 1. The assumption that a/b is irreducible simply means that the fraction representing the rational number is in simplest terms.
2. Sal then uses the expression 2b^2 = a^2 to show that a must be even.
( 2b^2 is an even number because it has a factor of 2.
So a^2 is also even because it equals 2b^2.
But a^2 is the same as a*a ---- and only even numbers can be multiplied by themselves to produce another even number.
Therefore, a is an even number.)
3. Sal then uses the expression 2b^2 = (2k)^2 in a similar way to show that b must be even.
4. Because both a and b are even, that means they both have a common factor of 2. (Note that is not the fraction a/b that is even, it is the individual numbers a and b.)
5. But, based on the initial assumption that a/b was irreducible, it is impossible for a and b to have a common factor.
6. Therefore, the assumption that sqrt2 was rational is incorrect. It is therefore irrational.
• Is a square root of any prime number always irrational?
• Yes. Think about the definition of prime number. It doesn't have any factors other than itself and 1. So there are no repeating numbers in its factorization thus its square root must be irrational
• How is (6)/(square root of 2) equal to 3 square root of 2 ?
• You multiply both the numerator & denominator by sqrt(2) and get
6 sqrt(2) / sqrt(2^2)
The denominator simplifies to sqrt(4) = 2
6 sqrt(2) / 2
Then reduce the fraction by the common factor of 2 to get:
3 sqrt(2)

Hope this helps.
• Can't the square root so two be expressed as a rational number as square root of two by. One.
• Good try! But that wouldn't be a rational number.
No rational number can include an irrational number as any part of it --- only integers and other rational numbers (like fractions and decimals).
• I have a question. If we take root 4 as the number and then try this method , then will we be able to avoid the contradiction? Assuming that we never write root 4 as 2?
• √4 = a/b where a, b are co-prime
⇒4 = a²/b²
⇒a² = 4b²
⇒a² has a factor of 4
⇒a has a factor of 2, i.e., a = 2k where k is an integer
Now, putting a = 2k, we get
(2k)² = 4b²
⇒4k² = 4b²
⇒b² = k²
⇒√b² = √k² (taking the square root on both sides)
⇒b = k
here, you may think that b has a factor of 1
But, it may have 2 as one of its factor since k can be either odd or even
So, this method doesn't work for rational numbers like √4.