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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
