Need up to 30 seconds to load.

here's an extra option for you kelsey's

even going to write this one down when

we're asking to find the sum

the sum is going to be

s of n equals n divided by 2 times a sub

1 plus a sub n

okay that was the sum formula you guys

should already have this written down

that's what we did last class

um because that's what we're going to

use

here is the problem i what i did was i

said all right here's the problem you

guys will see i broke up the problem of

the components of some notation

this is your formula

this is where you start

this is what sigma represents and here

is your end all right now i'll explain

my way through it once i divide because

when you guys get a test you're just

going to look like this and it's gonna

say hey you know question number one and

it's gonna look like that it's gonna say

what's the answer and so what you're

gonna have to know is oh they're asking

me for the sum

not right now i'm explaining something

that we really need to make sure we over

so we have s of n equals n divided by 2

times a sub 1 plus n a sub n do we know

what the first number is of our sequence

do we even have a sequence no no well we

only have a formula this explicit

formula and if you guys remember

three which represents the end

okay

so if you guys remember how we wrote

explicit formulas

we wrote explicit formulas as like a sub

n right it could have been arithmetic it

could have been geometric it doesn't

really matter

this is like your your explicit formula

can also be written as this oops where n

where is this x i'm using

sorry

sorry there's no x we're using n's here

n represents the number in sequence

sorry i got mistaken so can we figure

out what the first term is though once

we know what the formula is yes yeah you

just plug in the first number right

did everybody see that

does everybody see that yeah okay now

the next thing mario

i'm really having a hard time i really

am the next thing is what is our last

term

the last term is well what is the last

term it ends with three can we find out

what a sub 3 is

okay

yep

so now you just plug 3 in front 7 times

3 is 21 minus 1 is 20.

so kelsey you're not giving it a shot

either

all

right i thought you guys would try

mario still not trying

so now we have s of three

we look at this we say all right well

how many terms are we solving we have

starting with one we're ending at three

so we are doing

three terms

do we know what a sub one is

six do we know what a sub three is which

is because we're trying to find the sum

of three so this is really 3

and that's 3.

so we're really trying to do ace of a

sub n which is a sub 3. so a sub 3 we

know is 20.

so therefore s of 3 is equal to 3 halves

times 26. how do you multiply a fraction

times a whole number

you put a 1 over the 26

and then you just multiply straight

across well also it's helpful to

sometimes simplify as well so i could

divide the 2 into the 26 which would

give you

13 right i divide 2 into 26 across

so therefore now i just have 3 times 13

which is

s of 3

equals 39.

now that's it