Did you know that numbers can be polite? In math, a polite number is any number we can write as the sum of two or more consecutive positive whole numbers.
(Consecutive means numbers that come one right after another in the counting sequence.)
For example, five is a polite number, because we can write it as the sum of two consecutive numbers:
5 = 2 + 3
Nine is a doubly polite number, because we can write it two ways:
9 = 4 + 5
9 = 2 + 3 + 4
And fifteen is an amazingly polite number. We can write fifteen as the sum of consecutive numbers in three ways:
15 = 7 + 8
15 = 4 + 5 + 6
15 = 1 + 2 + 3 + 4 + 5
How many other polite numbers can you find?
What Do You Notice?
Are all numbers polite?
Or can you find an impolite number?
Can you make a collection of polite and impolite numbers? Find as many as you can.
How many different ways can you write each polite number as a sum of consecutive numbers?
What do you notice about your collection of polite and impolite numbers?
Can you think of a way to organize your collection so you can look for patterns?
What Do You Wonder?
Make a conjecture about polite or impolite numbers. A conjecture is a statement that you think might be true.
For example, you might make a conjecture that “All odd numbers are…” — How would you finish that sentence?
Make another conjecture.
Can you make at least five conjectures about polite and impolite numbers?
What is your favorite conjecture? Does thinking about it make you wonder about numbers?
Can you think of any way to test your conjectures, to know whether they will always be true or not?
Real Life Math Is Social
This is how mathematics works. Mathematicians play with numbers, shapes, or ideas and explore how those relate to other ideas.
After collecting a set of interesting things, they think about ways to organize them, so they can look for patterns and connections. They make conjectures and try to imagine ways to test them.
And mathematicians compare their ideas with each other. In real life, math is a very social game.
So play with polite and impolite numbers. Compare your conjectures with a friend.
Share your ideas in the comments section below.
And check out the list of student conjectures at the Ramblings of a Math Mom blog.
CREDITS: Numbers photo (top) by James Cridland via Flickr (CC BY 2.0). I first saw this activity at Dave Marain’s Math Notations blog, and it’s also available as a cute printable Nrich poster. For a detailed analysis, check out Wai Yan Pong’s “Sums of Consecutive Integers” article.
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