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Let us now study a very
important counting problem,

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the problem of counting
combinations.

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What is a combination?

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We start with a set
of n elements.

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And we're also given a
non-negative integer, k.

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And we want to construct or
to choose a subset of the

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original set that has
exactly k elements.

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In different language, we want
to pick a combination of k

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elements of the original set.

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In how many ways can
this be done?

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Let us introduce
some notation.

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We use this notation here, which
we read as "n-choose-k,"

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to denote exactly the quantity
that we want to calculate,

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namely the number of subsets
of a given n-element set,

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where we only count
those subsets that

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have exactly k elements.

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How are we going to calculate
this quantity?

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Instead of proceeding directly,
we're going to

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consider a somewhat different
counting problem which we're

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going to approach in two
different ways, get two

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different answers, compare
those answers, and by

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comparing them, we're going to
get an equation, which is

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going to give us in the end,
the desired number.

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The alternative problem
that we're going

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to use is the following.

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We start, as before, with
our given set that

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consists of n elements.

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But instead of picking a subset,
what we want to do is

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to construct a list, an
ordered sequence, that

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consists of k distinct elements
taken out of the

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original set.

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So we think of having k
different slots, and we want

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to fill each one of those slots
with one of the elements

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of the original set.

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In how many ways can
this be done?

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Well, we want to use the
counting principle.

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So we want to decompose this
problem into stages.

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So what we can do is to choose
each one of the k items that

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go into this list
one at a time.

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We first choose an item that
goes to the first position, to

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the first slot.

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Having used one of the items in
that set, we're left with n

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minus 1 choices for the
item that can go

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into the second slot.

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And we continue similarly.

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When we're ready to fill the
last slot, we have already

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used k minus one of the items,
which means that the number of

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choices that we're going to
have at that stage is n

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minus k plus 1.

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At this point, it's also
useful to simplify that

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expression a bit.

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We observe that this is the same
as n factorial divided by

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n the minus k factorial.

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Why is this the case?

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You can verify that this is
correct by moving the

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denominator to the other side.

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And when you do that, you
realize that you have the

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product of all terms from n
down to n minus k plus 1.

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And then you have the product of
n minus k going all the way

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down to one.

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And that's exactly the product,
which is the same as

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n factorial.

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It's a product of all integers
from n all the way down to 1.

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So this was the first method
of constructing the

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list that we wanted.

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How about a second method?

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What we can do is to first
choose k items out of the

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original set, and then take
those k terms and order them

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in a sequence to obtain
an ordered list.

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So we construct our ordered
list in two stages.

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In the first stage, how many
choices [do] we have?

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That's the number of subsets
with k elements out of the

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original set.

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This number, we don't
know what it is.

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That's what we're trying
to calculate.

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But we have a symbol for it.

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It's n-choose-k.

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How about the second stage?

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We have k elements,
and we want to

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arrange them in a sequence.

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That is, we want to form a

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permutation of those k elements.

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This is a problem that we have
already studied, and we know

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that the answer is
k factorial.

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According to the counting
principle, the number of ways

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that this two-stage construction
can be made is

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equal to the product of the
number of ways, number of

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options that we have in the
first stage times the number

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of options that we have
in the second stage.

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So this is one answer
for the number of

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possible ordered sequences.

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This is another answer.

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Of course, both of
them are correct.

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And therefore, they
have to be equal.

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And by using that equality, we
can now find a formula for

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this coefficient n-choose-k
simply by taking this k

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factorial factor and sending it
to the denominator of that

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expression.

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So by equating this expression
with that expression here, we

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find the final answer, which is
that the number of choices,

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n-choose-k, is equal to
this expression here.

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Now, this expression
is valid only for

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numbers that make sense.

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So n can be any integer, any
non-negative integer.

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And k, the only k's that make
sense, would be k's

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from 0, 1 up to n.

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You may be wondering about some
of the extreme cases of

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that formula.

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What does it mean for n to
be 0 or for k equal to 0?

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So let us consider now some of
these extreme cases and make a

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sanity check about
this formula.

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So this is the formula
that we have and

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that we want to check.

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The first case to consider
is the extreme case of

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n-choose-n.

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What does that correspond to?

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Out of a set with n elements,
we want to choose a subset

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that has n elements.

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There's not much of
a choice here.

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We just have to take all of the
elements of the original

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set and put them
in the subset.

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So the subset is the same
as the set itself.

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So we only have one
choice here.

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That should be the answer.

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Let's check it with
the formula.

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The formula gives
us n factorial

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divided by n factorial.

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And then, since k is equal to n,
here we get zero factorial.

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Is this correct?

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Well, it becomes correct
as long as we adopt the

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convention that zero factorial
is equal to 1.

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We're going to adopt this
convention and keep it

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throughout this course.

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Let's look at another extreme
case now, the

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coefficient n choose 0.

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This time let us start
from the formula.

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The formula tells us that this
should be n factorial divided

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by 0 factorial and divided by n
factorial, since the number

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k is equal to 0.

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Using the convention that we
have, this is equal to 1.

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So this is, again, equal to 1.

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Is it the correct answer?

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How many subsets of a given
set are there that have

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exactly zero elements?

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Well, there's only one subset
that has exactly 0 elements,

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and this is the empty set.

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So this explains this particular
answer and shows

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that it is meaningful and
that it makes sense.

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Now, let us use our
understanding of those

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coefficients to solve a somewhat
harder problem.

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Suppose that for some
reason, you want to

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calculate this sum.

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What is it going to be?

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One way of approaching this
problem is to use the formula

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for these coefficients,
do a lot of algebra.

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And if you're really patient
and careful, eventually you

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should be able to get
the right answer.

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But this is very painful.

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Let us think whether there's a
clever way, a shortcut, of

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obtaining this answer.

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Let us try to think what
this sum is all about.

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This sum includes this term,
which is the number of

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zero-element subsets.

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This number, which is the number
of subsets that have

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one element.

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And we keep going all the way to
the number of subsets that

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have exactly n elements.

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So we're counting zero-element
subsets, one-element subsets,

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all the way up to n.

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So what we're counting really
is the number of all subsets

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of our given set.

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But this is a number that
we know what it is.

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The number of subsets of
a given set with n

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elements is 2 to the n.

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So by thinking carefully and
interpreting the terms in this

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sum, we were able to solve
this problem very fast,

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something that would be
extremely tedious if we had

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tried to do it algebraically.

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For some practice with this
idea, why don't you pause at

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this point and try to solve a
problem of a similar nature?