If a number is a ratio of two integers (e.g., 1 over 10, -5 over 23, 1,543 over 10, etc.) then it is a rational number. Irrational numbers, when written as a decimal, they continue indefinitely without repeating. HowStuffWorks
When you hear the words “rational” and “irrational,” they might bring to mind the relentlessly analytical Spock in “Star Trek.” If you’re a mathematician, however, you probably think of ratios between integers versus square roots.
In the realm of mathematics, where words sometimes have specific meanings that are very different from everyday usage, the difference between rational and irrational numbers doesn’t have anything to do with emotions.
Remember the Word ‘Ratio’
“In remembering the difference between rational and irrational numbers, think one word: ratio,” explains Eric D. Kolaczyk. He’s a professor in the department of mathematics and statistics at Boston University and the director of the university’s Rafik B. Hariri Institute for Computing and Computational Science & Engineering.
“If you can write a number as a ratio of two integers (e.g., 1 over 10, -5 over 23, 1,543 over 10, etc.) then we put it in the category of rational numbers,” Kolaczyk says in an email. “Otherwise, we say it is irrational.”
You can express either a whole number or a fraction — parts of whole numbers — as a ratio, using an integer called a numerator on top of another integer called a denominator. You divide the denominator into the numerator. That can give you a number such as 1/4 or 500/10 (otherwise known as 50).
Understanding the Complexity of Irrational Numbers
Irrational numbers, in contrast to rational numbers, are pretty complicated. As Wolfram MathWorld explains, they cannot be expressed by fractions, and when you try to write them as a number with a decimal point, the digits just keep going on and on, without ever stopping or repeating a pattern.
So what sort of numbers behave in such a crazy fashion? Basically, ones that describe complicated things.
Pi
Perhaps the most famous irrational number is pi — sometimes written as π, the Greek letter for “p” — which expresses the ratio of the circumference of a circle to the diameter of that circle. As mathematician Steven Bogart explained in this 1999 Scientific American article, that ratio will always equal pi, regardless of the size of the circle.
Since Babylonian mathematicians attempted to calculate pi nearly 4,000 years ago, successive generations of mathematicians have kept plugging away, coming up with longer and longer strings of the decimal expansion with non-repeating patterns.
In 2019, Google researcher Emma Hakura Iwao managed to extend pi to 31,415,926,535,897 digits.
Some (But Not All) Square Roots
Sometimes, a square root — that is, a factor of a number that, when multiplied by itself, produces the number that you started with — is an irrational number, unless it’s a perfect square that’s a whole number, such as 4, the square root of 16.
One of the most conspicuous examples is the square root of 2, which works out to 1.414 plus an endless string of non-repeating digits. That value corresponds to the length of the diagonal within a square, as first described by the ancient Greeks in the Pythagorean theorem.
Why Do We Use the Words ‘Rational’ and ‘Irrational’?
“We do indeed typically use ‘rational’ to mean something more like based on reason or similar,” Kolaczyk says. “Its use in mathematics seems to have cropped up as early as the 1200s in British sources (per the Oxford English Dictionary). If you trace both ‘rational’ and ‘ratio’ back to their Latin roots, you find that in both cases the root is about ‘reasoning,’ broadly speaking.”
What’s clearer is that both irrational and rational numbers have played important roles in the advance of civilization.
While language probably dates back to around the origin of the human species, numbers came along much later, explains Mark Zegarelli, a math tutor and author who has written 10 books in the “For Dummies” series. Hunter-gatherers, he says, probably didn’t need much numerical precision, other than the ability to roughly estimate and compare quantities.
“They needed concepts like, ‘We have no more apples,'” Zegarelli says. “They didn’t need to know, ‘We have exactly 152 apples.'”
But as humans began to carve out plots of land to create farms, erect cities and manufacture and trade goods, traveling farther away from their homes, they needed a more complex math.
“Suppose you build a house with a roof for which the rise is the same length as the run from the base at its highest point,” Kolaczyk says. “How long is the stretch of roof surface itself from top to outer edge? Always a factor of the square root of 2 of the rise (run). And that’s an irrational number as well.”
The Role of Irrational Numbers in Modern Society
In the technologically advanced 21st century, irrational numbers continue to play a crucial role, according to Carrie Manore. She’s a scientist and a mathematician in the Information Systems and Modeling Group at Los Alamos National Laboratory.
“Pi is an obvious first irrational number to talk about,” Manore says via email. “We need it to determine area and circumference of circles. It’s critical to computing angles, and angles are critical to navigation, building, surveying, engineering and more. Radio frequency communication is dependent on sines and cosines which involve pi.”
Additionally, irrational numbers play a key role in the complex math that makes possible high-frequency stock trading, modeling, forecasting and most statistical analysis — all activities that keep our society humming.
“In fact,” Manore adds, “in our modern world, it almost makes sense to instead ask, ‘Where are irrational numbers not being used?'”
This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.
Frequently Asked Questions
What are irrational numbers?
Irrational numbers cannot be expressed as a ratio of two integers. When written as a decimal, they continue indefinitely without repeating.
What are examples of famous irrational numbers?
Pi (π), which describes the ratio of a circle’s circumference to its diameter, is perhaps the most famous irrational number. Another example is the square root of 2, which is approximately 1.414 followed by an endless string of non-repeating digits.
What is Euler’s number and is it an irrational number?
Euler’s number, often represented as “e,” is another well-known irrational number. It is roughly equal to 2.71828 and arises naturally in many areas of mathematics, especially in situations where growth is involved.
How does the golden ratio fit into the concept of irrational numbers?
The golden ratio, often denoted by the Greek letter φ (phi), is an irrational number roughly equal to 1.6180339887. It has unique mathematical properties and appears in various domains of art, architecture and nature due to its aesthetically pleasing proportions.
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