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Irrational Numbers

An irrational number is a number that can't be expressed by a fraction having integers in both its numerator and denominator. While their existence was once kept secret from the public for philosophical reasons, they are now well accepted, yet still surprisingly hard to prove on an individual basis. Please post all questions about irrational numbers, including the famous examples of π, e, and √2, into this category.

500 Questions

Prove that negative of an irrational number is an irrational number?

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Assume = a/b with positive integers a und b. Now, for some natural number n define the functions f and F as follows. Strictly speaking, f and F should each have n as an index as they depend on n but this would render things unreadable; remember that n is always the same constant throughout this proof. Let f(x) = xn(a-bx)n/n! and let F(x) = f(x) + ... + (-1)jf(2j)(x) + ... + (-1)nf(2n)(x) where f(2j) denotes the 2j-th derivative of f. Then f and F have the following properties: f is a polynomial with coefficients that are integer, except for a factor of 1/n! f(x) = f(-x) 0

Why is the square root of three an irrational number?

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I'm assuming that you mean 'square root'. Yes, this sum is irrational. So are each of the two numbers alone. A simple proof can be done by writing x=square root 2 + square root 3 and then "squareing away" the square roots and then use the rational roots theorem. The sum or difference of two irrational number need not be irrational! Look at sqrt(2)- sqrt(2)=0 which is rational.

What is 40 in French?

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'Quarante' is the french word for forty.

Which of the following are square roots of 36?

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The square roots of 36 are +6 & -6

Remember +6 x + 6 = (+)36

Also

-6 x -6 = (+)36 Double minus becomes positive, so square roots must show both plus and minus answers.

= +36

Is the root of 7 irrational?

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Yes, and here's the proof:

Let's start out with the basic inequality 4 < 7 < 9.

Now, we'll take the square root of this inequality:

2 < √7 < 3.

If you subtract all numbers by 2, you get:

0 < √7 - 2 < 1.

If √7 is rational, then it can be expressed as a fraction of two integers, m/n. This next part is the only remotely tricky part of this proof, so pay attention. We're going to assume that m/n is in its most reduced form; i.e., that the value for n is the smallest it can be and still be able to represent √7. Therefore, √7n must be an integer, and n must be the smallest multiple of √7 to make this true. If you don't understand this part, read it again, because this is the heart of the proof.

Now, we're going to multiply √7n by (√7 - 2). This gives 7n - 2√7n. Well, 7n is an integer, and, as we explained above, √7n is also an integer; therefore, 7n - 2√7n is an integer as well. We're going to rearrange this expression to (√7n - 2n)√7, and then set the term (√7n - 2n) equal to p, for simplicity. This gives us the expression √7p, which is equal to 7n - 2√7n, and is an integer.

Remember, from above, that 0 < √7 - 2 < 1.

If we multiply this inequality by n, we get 0 < √7n - 2n < n, or, from what we defined above, 0 < p < n. This means that p < n and thus √7p < √7n. We've already determined that both √7p and √7n are integers, but recall that we said n was the smallest multiple of √7 to yield an integer value. Thus, √7p < √7n is a contradiction; therefore √7 can't be rational, and so must be irrational.

Q.E.D.

Do irrational numbers never repeat in a pattern or just never repeat?

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They never repeat in any pattern.

If they never repeated, you could have at most 10 digits after the decimal point and therefore the decimal representation would be terminating.

Is 4.2 an irrational or rational number?

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It is rational because it can be expressed as a fraction which is 21/5

What is a number fact?

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'Zero' and 'one' are the only numbers that are written the same in any base.

Is 2 over 4 an irrational or a rational number?

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The official definition of a rational number is: Any fraction with whole numbers on top and bottom.

2/4 certainly meets that description.

Pi is a fraction as circumference by diameter p by q form is a rational number then how is pi a irrational number?

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A number is said to be rational if it can be expressed as a ratio of two [finite] integers [the second of which is non-zero].

Here, if pi is to be rational, both p and q must be integers. If p is an integer then q will be irrational and if q is an integer, p will be irrational. They can never both be rational and so the ratio definition cannot be satisfied.

Is 9 over 8 an irrational?

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No, it is rational.

Is 4.50 irrational?

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No, it is rational.

Show that set R of real numbers is a group with respect to multiplication?

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Closure: If x and y are any two elements of Rthen x*y is an element of R.

Associativity: For and x, y and z in R, x*(y*z) = (x*y)*z and so, without ambiguity, this may be written as x*y*z.

Identity element: There exists an element 1, in R, such that for every element x in R, 1*x = x*1 = x.

Inverse element: For every x in R, there exists an element y in R such that x*y = y*x = 1. y is called the inverse of x and is denoted by x^-1.

The above 4 properties determine a group.

Can a number be a rational number and not an integer?

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Yes, just look at decimals.

Note: integers are a subset of rational numbers.

Is .786666666666... rational or irrational?

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Assuming that the notation "666..." represents the number with infinitely many 6s, the number is rational.

Why square root of 10 is irrational?

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Because it can't be expressed as a fraction