We're asked how many toothpicks will be needed to
form the 50th figure in this sequence?
And they've drawn the first four figures in the sequence.
This is the first, the second, the third.
Now, let's see what's happening here.
So in this first figure, we have this nice little house
that they've made out of toothpicks, and it has one,
two, three, four, five, six toothpicks.
So the first figure in our sequence has six toothpicks.
Now, what happens in the second figure here?
Well, it looks just like our first figure.
We have our one, two, three, four, five, six toothpicks,
but then they add some toothpicks.
It looks like they added one, two, three, four, five,
toothpicks.
So it's 6 plus 5 or 11 toothpicks.
So let me write this down.
It is the 6 toothpicks plus 5 more, which is equal to 11
toothpicks.
And then when we go to the third figure, it looks just
like the second figure, so it has the 11 toothpicks, so the
11 toothpicks are one, two, three, four, five, six, seven,
eight, nine, ten, eleven, just like the second figure.
And then they add some more.
They add one, two, three, four, five.
So it looks like every time, when you draw part of a
another house, because it can share a wall with the previous
house or share a toothpick with the previous set of
houses, you add five toothpicks.
So for the third term in our sequence, we had 11 and when
we had our second term, or our two houses, and
then we add five more.
So to our 50th term, so 11 plus 5 is going to be 16.
And then for the fourth term, it's going to be 16 plus 5,
which is equal to 21.
Now, there's a couple of ways to think about it.
You can think about however many more you are than 1.
So let's say you are in the nth term of the sequence.
If you are in the nth term, however more you are than 1,
so if you're the nth term, you're going to be n minus 1
more than 1.
If that is confusing, we'll do it with real numbers, so it
gets a little bit more tangible.
So the nth term, you are n minus 1 more or greater than--
I'll just say more-- than 1.
For example, if n is 2, you are 1 more than 1.
If n is 3, you are 3 minus 1, which is 2 more than 1.
So however much more you are than 1, you
multiply that by 5.
We're 2 more than 1, so we add 10 to the number of toothpicks
we have here.
We're 3 more than 1 here, so we add 15 to the number of
toothpicks we have there.
So you could say that for the nth term, the number of
toothpicks is equal to the number you're more than 1, < n
minus 1 times 5.
That's how much you're going to add above and beyond the
amount of toothpicks in just the first
sequence, so times 5.
n minus 1 times 5 plus the number of toothpicks that you
would just have in the first sequence or
just this one house.
And we already counted that: plus 6.
So that's one way to think about it.
And if this looks complicated, you just say, well, look, if I
put a 4 here, I'm 3 more than 1, so 4 minus 1 is 3, so
that'll be 3 times 5, which is 15.
And then you add the number of toothpicks in 1 and
then you get 6.
Now another way, and many of you all might find this easier
to think about, is even in 1, you could imagine a term
here-- let me do it this way.
You can imagine a term here that they didn't draw.
You can imagine a 0th term.
Let me just draw it here.
Imagine a 0th term, and the 0th term would just be kind of
a left wall of the house, or in this case, the left
toothpick of a house.
And then the first one, you're adding 5 toothpicks to that.
In the second one, you're adding 5 toothpicks to that.
And when you think about it this way, it actually becomes
a little simpler to think in terms of n.
Here, you could say, well, the nth term-- let me do this in a
different color.
You could say that the nth term is going to have-- or
maybe we should say number of toothpicks in nth figure is
going to be equal to 1.
So in the 0th figure, which I just made up, you have at
least 1 toothpick, and then whatever term in the sequence
you are, you multiply that times 5 and you add that to
the number of toothpicks in the 0th figure, so it's 1 plus
5 times n, which is actually a simpler way to think about.
The first figure is going to be that 1 in the 0th figure
plus 5: 6 toothpicks.
Even in the 0th figure, it works out.
If you put a 0 here, 0 plus 1 is 1.
If you put the fourth figure here, 5 times 4 is
20 plus 1 is 21.
Now let's answer their question.
We need to figure out the toothpicks in the 50th figure
in the sequence.
Well, we just put 50 for n here.
So for the 50th figure, we could use either formula.
We have 1 plus 5 times 50.
5 times 50 is 250 plus 1 is equal to 251.
And now I said you could use either formula because they
should reduce to each other, if we did our math right or if
we deduced properly.
So this one up here, let's just verify this is the same
thing as this over here.
If we multiply 5 times n minus 1, we get 5n minus 5, right?
We just distributed the 5.
And then we have that plus 6.
Negative 5 plus 6 is plus 1.
So it's 5n plus 1 or 1 plus 5n.
Either way, your 50th figure is going to have 251.