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## Get ready for Precalculus

### Unit 6: Lesson 4

Modeling with sequences# Sequences word problems

CCSS.Math: , , , , , ,

Sal solves two word problems about modeling real-world situations with arithmetic and geometric sequences.

## Want to join the conversation?

- How is it 1.4?? shouldn't it be 0.4??(11 votes)
- The number of leaves is 40%
**more**than the number of leaves in the previous year,**not**40% of the number of leaves in the previous year. Increasing a number by 40% means adding 100% of the number plus 40% of the number, which gives a total of 140% of the number. So the number of leaves is multiplying by 1.4 each year.(30 votes)

- How come Sal writes 50-3(n-1) instead of 50-(3)^n-1 like we have been learning for geometric sequences?(9 votes)
- Because the difference between consecutive term values is a fixed quantity, that means that the sequence is an arithmetic one!(8 votes)

- Technically couldn't you say you're adding 40%?(7 votes)
- Well, adding 40% of x is the same as x + 40%x, or x + 0.4x, or 1.4x. It's easier to just write it as 1.4x- clearer and more concise.(8 votes)

- If we were to write it as a function though, its form would be g(n) = 50-3n. So when she has 0 guests, she would have all the favors and when someone comes, he gets three favors right away. This premise of delayed bestowal of favors in this problem is a little confusing. If g(n) denotes the number of favors before the nth guest has arrived, then that means that the host is waiting for the next guest to come before giving the previous guest his favors and I don't see how the question supposes that. It explicitly says: "she gave 3 party favors to each of her guests as they arrived", so I guess "as" should mean "as soon as" which means "instantly".(8 votes)
- Giannis,

1. Expand the formula and simplify - it should be**53 - 3n**. Now the maths makes sense.

2. To avoid repetition, read the commentary between Eric Allen Connor and David Severin. People are making this question way harder than it actually needs to be.

"as they arrived" would mean that 3 party favours were given when the person arrived at the door.

The 1st guest comes to the party and gets 3 party favours. Seo-Yun is left with 47 at the moment. When the 2nd guest comes, they get 3 as well. The 1st guest**doesn't**have to wait for the 2nd guest. Don't over complicate the question.(1 vote)

- Just saw these symbols in the explanation of an answer for sequence word answers

||| :-: | :-: | :-:

Can anyone help explain what these mean?

Thanks.(5 votes)- Here's a Wikipedia article on math symbols, I don't know if it has the symbols you want though.

https://en.wikipedia.org/wiki/List_of_mathematical_symbols(2 votes)

- why was 1.4 multiplied with 500 and even with the next no(2 votes)
- At2:30Sal says “to grow by 40% you’re going to multiply by 1.4." Okay, so why by 1.4? I’ll break it down and use 500 for my example.

Beginning the first year there were 500 leaves. We can determine that the count of 500 leaves would increase by 200 more leaves (40% of 500 = 200; 0.4 * 500 = 200) during the first year.

Okay. At the end of the first year (i.e. the start of the second year) we will have 500 (our initial count) plus 200 (what grew during the year). So we get:

500 + 200 [The amount of leaves when starting the second year]

500 + ( 0.4 * 500 ) [We calculated 200 with 0.4 * 500 (40% of 500)]

I have two terms that both have 500 so I can factor the 500 out of both. To help illustrate that I will make an equivalent statement:

( 500 * 1 ) + ( 500 * 0.4 ) [ 500 * 1 is 500. ]

Now I can clearly use the distributive property to factor 500 out of both terms giving me:

500 * ( 1 + 0.4 )

500 * 1.4 [adding the two terms]

That’s how we get 1.4.

Now the second part of your question. Why do we multiply the next year’s number of leaves by 1.4? [Implying multiplying each of the following years.]

Simply put each year’s number of leaves grows an additional 40% from the previous year’s total of leaves. Just as we multiplied the first year by 1.4, we need to do it to the second year, the third year, … as the number of leaves grows each year.

I hope this helps.(8 votes)

- 40% more = 1.4

40 % less = -1.4

40% multiplied = 0.4

That's the way to do it, right?(3 votes)- Not quite 40% more is 1 + . 4 = 1.4 *
*__correct*

40 % less is 1 - .4 = .6 * __

40% of (which is better wording than multiplied, but means the same) = .4 * ___

40% off of is same as 40% less

2 out of 3 ain't bad(5 votes)

- What if she had zero guest?(3 votes)
- Sequences only work for positive integers i.e. their domain is limited to positive integers , so 0 guests is not possible(3 votes)

- In the second problem, shouldn't the equation be 50-3(n) or 47-3(n-1) because the question states what are the number of favors the host has
*before*the nth guest arrives?

For example, when n=0, the favors remaining are 50**because**there are no guests who have arrived. It's just the host and his 50 favors. When n=1 (i.e. when the first guest arrives) the guest receives three favors, which leaves 47 remaining favors. So when n=2 the favors remaining are 44. Etc.(3 votes)- Sal emphasizes that fact that it is the number of party favors she has before the guest arrives. So before the 1st guest arrives, there are 50, before the second guest arrives, there are 47, etc.(2 votes)

- How Do You know if its a Geometric sequence or a Arithmetic.(2 votes)
- Arithmetic sequences have a common difference (add or subtract same number every time). Example 1, 3, 5, 7, 9, ... (+ 2) or 14, 11, 8, 5, 2, -1, ... (- 3)

Geometric sequences have a common ratio (multiply or divide same number every time). Example 1, 3, 9, 27, ... (x3) or 16, 8, 4, 2, 1, 1/2, 1/4, ... (* 1/2 which is divide by 2).(3 votes)

## Video transcript

- [Voiceover] Mohamed
decides to track the number of leaves on the tree in
his backyard each year. The first year, there were 500 leaves. Each year thereafter, the number of leaves was 40% more than the year before. Let n be a positive integer,
and let f of n denote the number of leaves on
the tree in Mohamed's back yard in the nth year
since he started tracking it. The expression f of n defines a sequence. What kind of sequence is f of n? So, some of you might be able to think about this in your head. Each successive year we're growing by 40%, that's the same thing
as multiplying by 1.4. Each successive term we're multiplying or dividing by the same number. Well, that's going to be geometric. Let's make that a little
bit more tangible, just in case. So, let's make a little table here. So, table. So, this is n and this is f of n. So when n is equal to one, the first year, n equals
one, there were 500 leaves. F of n is 500. Now, when n is equal to two we're going to grow by 40%, which is the same thing as multiplying by 1.4. So 500 times 1.4, let's
do 40% of 500 is 200, so we're going to grow by 200, so we're going to go to 700. Then in year three, we're
going to grow by 40% of 700, which is 280, so
it's going to grow to 980. Notice it's definitely not
an arithmetic sequence. An arithmetic sequence, we would be adding or subtracting the same amount every time, but we're not. Here, from 500 to 700, we grew by 200, and then from 700 to 980, we grew by 280. Instead, we're multiplying or dividing by the same amount each time. In this case, we're multiplying by 1.4, by 1.4 each time. So we are clearly geometric. Depending on your answer
to the question above, the recursive definition
of the sequence can have one of the following two forms. Well this is the arithmetic form, which we know isn't the case,
so it's going to be in the geometric form. And then they ask us, what are the values of the parameters A
and B for the sequence? So we have our base case
here, f of n is going to be equal to A when n is equal to one. Well, we know that when n equals one, we had 500 leaves on the
tree so A, this A over here, is 500, so A is 500, and then if we're not in the base case for any other year, we are going to have,
let's see, it's going to be the previous year, the
previous year times what? It's going to be the
previous year grown by 40%, to grow by 40%, you're
going to multiply by 1.4 so B is going to be 1.4. You take the previous
year and you multiply by 1.4 for any other year, any year other than n equals one. So, B is equal to 1.4, and we're done. Let's do another example. This is strangely fun. All right, so this says:
Seo-Yun hosted a party. She had 50 party favors
to give away, and she gave away three party
favors to each of her guests as they arrived at the party. Let n be a positive integer,
and let g of n denote the number of party
favors Seo-Yun had before the nth guest arrived. All right, actually, before I
even look at these questions, let me make a table here
because they're saying before the nth guest. I want to make sure I'm
understanding this properly. So this is n, and then this is going to be g of n, right over here. So, when n is equal to one, when n is equal to one,
g of n is going to be, or g of one is going to be
the number of party favors Seo-Yun had before the first guest. Well, before the first guest,
she had 50 party favors. She had 50 party favors. Now, the second guest comes. Now the number of party
favors she had before the second guest, well,
she had to give three to the first guest, so she's now going to have 47 party favors. Now, when n is equal to three, how many party favors did she have before the third guest? Well, she would've had
to give party favors to the first and second
guest, who each got three. So, she would have 44, and
I think you see the pattern. For every time n, when n
equals one, g of n is 50, and every time we increase n by one, every time we increment
n, we are increasing g of n by plus three, by
minus three, I should say because she's giving away
party favors, minus three. Minus three. So because the difference
between successive terms is the same, we know this
is an arithmetic sequence. This is an arithmetic
sequence and then they say write an explicit
formula for the sequence. So let's think about this. Let's see, g of n is
going to be equal to... Let's see, we're going to start at 50 then we're going to subtract
three, and let's think, do we subtract three times n or is it...? Let's see, for the first guest, we subtract three zero times. For the second guest,
we subtract three once. For the third guest, we
subtract three twice. So, for the nth guest, we're
going to subtract three n minus one time. Notice, for the nth guest,
we subtracted three twice. The second guest, subtracted three once. First guest, subtracted three zero times, so this works out. For the first guest, we would
subtract three zero times, and so g of one would be 50. We can see that this is
consistent for all of these. So I could write 50 minus
three times n minus one, and I really recommend
making the table here just so you make sure
you get the n minus one or the n right and it all gels.