Skip to 0 minutes and 7 secondsPAULA KELLY: So let's have a look at another number sequence. We have here some triangle numbers. And we'll show you with some blocks to see how that looks.

Skip to 0 minutes and 15 secondsMICHAEL ANDERSON: OK.

Skip to 0 minutes and 16 secondsPAULA KELLY: So what we have here is our fourth triangle number. Now, a triangle shape to our triangle number.

Skip to 0 minutes and 23 secondsMICHAEL ANDERSON: Yeah.

Skip to 0 minutes and 23 secondsPAULA KELLY: You may notice it has a base of 4.

Skip to 0 minutes and 25 secondsMICHAEL ANDERSON: Mm-hmm.

Skip to 0 minutes and 26 secondsPAULA KELLY: So our fourth triangle number-- we could do with knowing how many blocks are in this fourth.

Skip to 0 minutes and 31 secondsMICHAEL ANDERSON: Yeah. So we can probably just count those.

Skip to 0 minutes and 33 secondsPAULA KELLY: Yeah.

Skip to 0 minutes and 34 secondsMICHAEL ANDERSON: So we've got 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. So this is our fourth triangle number. It's a height of 4 and a width of 4. And to make it, we need 10 blocks.

Skip to 0 minutes and 46 secondsPAULA KELLY: OK.

Skip to 0 minutes and 46 secondsMICHAEL ANDERSON: So what's the triangle number before that one?

Skip to 0 minutes and 49 secondsPAULA KELLY: So as our fourth triangle number has a base of 4, height of 4, using our logic, our third triangle number has a base of 3, height of 3.

Skip to 0 minutes and 58 secondsMICHAEL ANDERSON: Oh, so we've essentially just taken away this bottom row?

Skip to 1 minute and 1 secondPAULA KELLY: Absolutely. Yeah.

Skip to 1 minute and 3 secondsMICHAEL ANDERSON: And if we look at the number of blocks we need to make this one, it's just 1, 2, 3, 4, 5, 6.

Skip to 1 minute and 9 secondsPAULA KELLY: OK. So if we continue to it backwards, we have our fourth, our third, our second triangle number. Let's keep our pattern going.

Skip to 1 minute and 17 secondsMICHAEL ANDERSON: Mm-hmm.

Skip to 1 minute and 18 secondsPAULA KELLY: It's just going to have a base of 2, a height of 2.

Skip to 1 minute and 21 secondsMICHAEL ANDERSON: Yep.

Skip to 1 minute and 21 secondsPAULA KELLY: But this time, fewer blocks. We have just 3 blocks this time.

Skip to 1 minute and 24 secondsMICHAEL ANDERSON: OK.

Skip to 1 minute and 25 secondsPAULA KELLY: OK. Finally, our first triangle number. As we've done all the way along, we've taken away 4, taken away 3, taken away 2. So for our first triangle number, just 1 block.

Skip to 1 minute and 38 secondsMICHAEL ANDERSON: Doesn't look much like a triangle.

Skip to 1 minute and 39 secondsPAULA KELLY: [LAUGHS] It doesn't do. But we can see from our pattern how it builds up into a triangle, and we're seeing how we've gone backwards from the fourth triangle number right down to the first one.

Skip to 1 minute and 48 secondsMICHAEL ANDERSON: So usually, with these sequences, we normally work up. So we'd start with 1, and then we'd add something, and add something, and add something again. So how does this one grow, working upwards?

Skip to 1 minute and 59 secondsPAULA KELLY: So if we notice, we start from our first triangle number. We have just 1. We notice with our numbers, we add on 2. But showing it with our block makes it even clearer. We can see we've added an extra row. That's an extra 2 blocks.

Skip to 2 minutes and 12 secondsMICHAEL ANDERSON: OK. So to get from 1 to 3, we add 2.

Skip to 2 minutes and 19 secondsPAULA KELLY: OK-doke. OK. So looking at our pattern again, we've added on an extra row at the bottom. We've added on an extra 3 blocks.

Skip to 2 minutes and 27 secondsMICHAEL ANDERSON: OK. So we're adding 3 on this time. So it's not an arithmetic sequence, because this time we're adding on 3 instead of 2. The differences are different.

Skip to 2 minutes and 38 secondsPAULA KELLY: Exactly. So for an arithmetic sequence, we have to go up by the same amount each time. So for triangle numbers, we can see now, it's not necessarily an arithmetic sequence.

Skip to 2 minutes and 47 secondsMICHAEL ANDERSON: OK. And to get from 6 to 10, we added this last row in, so we're going to add 4. So it seems to me that the amount we're adding on each time is increasing by 1 each time. So to get the fifth triangle number, we probably add 5. And then add 6, and then add 7, and add 8. And keep on going to get the next term, and the next term, and the next turn.

Skip to 3 minutes and 6 secondsPAULA KELLY: Yeah. Forever and ever and ever. OK. Yes.

Skip to 3 minutes and 8 secondsMICHAEL ANDERSON: OK. So now we know how to get from one term to the next term, let's see if there's a general rule for the triangle numbers.

Triangle numbers

Triangle numbers are those whole numbers such that if that number is represented by that many dots we can arrange the dots in the shape of a triangle.

Paula and Michael explain how we can use interlocking cubes to show how triangle numbers grow which leads to a term to term rule. In the second part of the video, the cubes are again used to show how a position to term rule can be used.

Explain

Once you have watched the video explain how to create triangle numbers using the term to term rule.

Then explain how to make triangle numbers using the position to term rule.

If you were asked to find the hundredth triangle number which rule will you use? Explain your choice.

Problem worksheet

Now complete question 9 from this week’s worksheet

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This video is from the free online course:

Maths Subject Knowledge: Understanding Numbers

National STEM Learning Centre