Difference between revisions of "2011 AIME II Problems/Problem 6"
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==Solution 2== | ==Solution 2== | ||
− | Let us consider our quadruple (a,b,c,d) as the following image | + | Let us consider our quadruple <math>(a,b,c,d)</math> as the following image <math>xaxbcxxdxx</math>. The location of the letter <math>a</math>, <math>b</math>, <math>c</math>, <math>d</math> represents its value and <math>x</math> is a place holder. Clearly the quadruple is interesting if there are more place holders between <math>c</math> and <math>d</math> than there are between <math>a</math> and <math>b</math>. |
+ | |||
+ | <math>0</math> holders between <math>a</math> and <math>b</math> means we consider <math>a</math> and <math>b</math> as one unit <math>ab</math> and <math>c</math> as <math>cx</math> yielding <math>\binom83 = 56</math> ways; | ||
+ | |||
+ | <math>1</math> holder between <math>a</math> and <math>b</math> means we consider <math>a</math> and <math>b</math> as one unit <math>axb</math> and <math>c</math> as <math>cxx</math> yielding <math>\binom 63 = 20</math> ways; | ||
+ | |||
+ | <math>2</math> holders between <math>a</math> and <math>b</math> means we consider <math>a</math> and <math>b</math> as one unit <math>axxb</math> and <math>c</math> as <math>cxxx</math> yielding <math>\binom43 = 4</math> ways. | ||
+ | |||
+ | Since there cannot be <math>3</math> holders between <math>a</math> and <math>b</math> so our total is <math>56+20+4=\fbox{080}</math>. | ||
==Solution 3 (<i>Slightly</i> bashy)== | ==Solution 3 (<i>Slightly</i> bashy)== | ||
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==Solution 4 (quick)== | ==Solution 4 (quick)== | ||
− | Notice that if <math>a+d>b+c</math>, then <math>(11-a)+(11-d)<(11-b)+(11-c)</math>, so there is a | + | Notice that if <math>a+d>b+c</math>, then <math>(11-a)+(11-d)<(11-b)+(11-c)</math>, so there is a [[bijection]] between the number of ordered quadruples with <math>a+d>b+c</math> and the number of ordered quadruples with <math>a+d<b+c</math>. |
− | Quick counting gives that the number of ordered quadruples with <math>a+d=b+c</math> is 50. | + | Quick counting gives that the number of ordered quadruples with <math>a+d=b+c</math> is 50. To count this, consider our numbers <math>1, 2, 3, 4, 5, 6, 7, 8, 9, 10</math>. Notice that if, for example, <math>a+d=b+c=8</math>, that the average of <math>a,d</math> and <math>b,c</math> must both be <math>4</math>. In this way, there is a symmetry for this case, centered at <math>4</math>. If instead, say, <math>a+d=b+c=7</math>, an odd number, then there is symmetry with <math>(a,d);(b,c)</math> about <math>3.5</math>. Further, the number of cases for each of these centers of symmetry correspond to a triangular number. Eg centered at <math>2.5,3,8,8.5</math>, there is <math>1</math> case for each and so on, until centered at <math>5.5</math>, there are <math>10</math> possible cases. Adding these all, we have <math>2(1+3+6)+10=50</math>. |
Thus the answer is <math>\frac{\binom{10}{4}-50}{2} = \boxed{080}.</math> | Thus the answer is <math>\frac{\binom{10}{4}-50}{2} = \boxed{080}.</math> | ||
+ | |||
+ | ==Solution 5== | ||
+ | |||
+ | Think about a,b,c,and d as distinct objects, that we must place in 4 of 10 spaces. However, in only 1 of 24 of these combinations, will the placement of these objects satisfy the condition in the problem. So we know the total number of ordered quadruples is <math>(10*9*8*7/24)=210</math> | ||
+ | |||
+ | Next, intuitively, the number of quadruples where <math>a+d>b+c</math> is equal to the number of quadruples where <math>a+d<b+c</math>. So we need to find the number of quadruples where the two quantities are equal. To do this, all we have to do is consider the cases when <math>a-d</math> ranges from 3 to 9. It would seem natural that a range of 3 would produce 1 option, and a range of 4 would produce 2 options. However, since b and c cannot be equal, a range of 3 or 4 produces 1 option each, a range of 5 or 6 produces 2 options each, a range of 7 or 8 produces 3 options each, and a range of 9 will produce 4 options. In addition, a range of n has 10-n options for combinations of a and d. Multiplying the number of combinations of a and d by the corresponding number of options for b and c gives us 50 total quadruplets where <math>a+d=b+c</math>. | ||
+ | |||
+ | So the answer will be <math>\frac{210-50}{2} = \boxed{080}.</math> | ||
+ | |||
+ | |||
+ | |||
+ | == Solution 6 == | ||
+ | |||
+ | |||
+ | Let <math>b = a+x</math> and <math>c = a+x+y</math> and <math>d = a+x+y+z</math> for positive integers <math>x,y,z.</math> In order to satisfy the other condition we need <math>z > x</math> so we let <math>z = x+k.</math> Now the only other condition we need to satisfy so <math>a+2x+y+k \le 10.</math> This condition can be transformed into <math>a+2x+y+k+b = 11</math> for positive <math>a,x,y,k,b.</math> Now we use generating functions to finish. We find the generating function of the whole expression is <math>(x + x^2 + \cdots)^4 \cdot (x^2+x^4 + \cdots)</math> and we are looking for the <math>x^{11}</math> coefficient. This simplifies to finding the <math>x^5</math> coefficient of <math>(1+x+\cdots)^4 \cdot (1+x^2+\cdots) =\frac{1}{(1-x)^4} \cdot\frac{1}{1+x}.</math> Now this expression simplifies to <cmath>\left(\binom{4}{4}+\binom{5}{4} + \cdots +\binom{4+k}{4}x^k\right)(1-x+x^2-x^3 \cdots).</cmath> The <math>x^5</math> coefficient ends up to be <math>\binom{9}{4} -\binom{8}{4} +\binom{7}{4} -\binom{6}{4} +\binom{5}{4} -\binom{4}{4} = 126 - 70 + 35 - 15 + 5 - 1 = \boxed{080}.</math> | ||
+ | |||
+ | == Solution 7 (<i>Slightly Slower</i>)== | ||
+ | |||
+ | First, let <math>a=1</math> and <math>d=10</math>. If <math>b=2</math>, then <math>c</math> can be from <math>3</math> to <math>8</math>. If <math>b=3</math>, then <math>4</math> to <math>7</math>. If <math>b=4</math>, then <math>c</math> is between <math>5</math> and <math>6</math>. We find a pattern that whenever <math>b</math> increases by <math>1</math>, when <math>a</math> and <math>d</math> are stationary, then the possible values of <math>c</math> decrease by 2, unless it gets to zero or negative, in which case that case ends. Counting up, we have <math>6+4+2=12</math> different possibilities when <math>a=1</math> and <math>d=10</math>. For <math>a=1</math> and <math>d=9</math>, <math>b=2</math>, then <math>c</math> can be from <math>3</math> to <math>7</math>. If <math>b=3</math>, then <math>c</math> can be from <math>4</math> to <math>6</math>, and so on. Notice that the possible values for each case of <math>b</math> gets one less than if <math>d</math> were one greater, unless that number is zero, in which it stays zero. We then use this pattern to find all the values: <math>12+9+6+4+2+1+9+6+4+2+1+6+4+2+1+4+2+1+2+1+1 \Rightarrow \\ 12\cdot1+9\cdot2+6\cdot3+4\cdot4+2\cdot5+1\cdot6= \\ 12+18+18+16+10+6=\boxed{\textbf{080}}.</math> | ||
+ | |||
+ | ~[[User:Sweetmango77|SweetMango77]] | ||
+ | |||
+ | === Note === | ||
+ | Note that the first value of each of the rows, if we arrange them based on values of <math>a</math>, is deleted after each row. | ||
+ | |||
+ | == Solution 8 == | ||
+ | Rearranging the equation obtains <math>b-a<d-c</math>. Let <math>a-0=e_0</math>, <math>b-a=e_3+e_1</math>, <math>c-b=e_2</math>, <math>d-c=e_3</math>, <math>11-d=e_4</math>. Add up all of these newly defined equations to obtain <math>e_0+e_1+e_2+2e_3+e_4=11</math>. Note that since all <math>e_n</math> were defined to be <math>e_n\ge1</math>, to form our stars and bars argument we can let <math>d_n+1=e_n</math> for all <math>n</math>. Then we obtain <math>d_0+d_1+d_2+2d_3+d_4=5</math> where <math>d_n</math> is nonnegative. Now, we can move the <math>2d_3</math> term to the other side and perform casework. | ||
+ | |||
+ | If <math>2d_3=0</math>: 5 objects for 4 variables -> <math>\binom{8}{3}</math> | ||
+ | |||
+ | If <math>2d_3=2</math>: 3 objects for 4 variables -> <math>\binom{6}{3}</math> | ||
+ | |||
+ | If <math>2d_3=4</math>: 1 object for 4 variables -> <math>\binom{4}{3}</math> | ||
+ | |||
+ | Adding all of these cases up, we get <math>56+20+4=\boxed{080}</math> as our requested answer. | ||
+ | |||
+ | ~sigma | ||
+ | |||
==See also== | ==See also== |
Latest revision as of 01:28, 16 January 2023
Contents
Problem 6
Define an ordered quadruple of integers as interesting if
, and
. How many interesting ordered quadruples are there?
Solution 1
Rearranging the inequality we get . Let
, then
is a partition of 11 into 5 positive integers or equivalently:
is a partition of 6 into 5 non-negative integer parts. Via a standard stars and bars argument, the number of ways to partition 6 into 5 non-negative parts is
. The interesting quadruples correspond to partitions where the second number is less than the fourth. By symmetry, there are as many partitions where the fourth is less than the second. So, if
is the number of partitions where the second element is equal to the fourth, our answer is
.
We find as a sum of 4 cases:
- two parts equal to zero,
ways,
- two parts equal to one,
ways,
- two parts equal to two,
ways,
- two parts equal to three,
way.
Therefore, and our answer is
Solution 2
Let us consider our quadruple as the following image
. The location of the letter
,
,
,
represents its value and
is a place holder. Clearly the quadruple is interesting if there are more place holders between
and
than there are between
and
.
holders between
and
means we consider
and
as one unit
and
as
yielding
ways;
holder between
and
means we consider
and
as one unit
and
as
yielding
ways;
holders between
and
means we consider
and
as one unit
and
as
yielding
ways.
Since there cannot be holders between
and
so our total is
.
Solution 3 (Slightly bashy)
We first start out when the value of .
Doing casework, we discover that . We quickly find a pattern.
Now, doing this for the rest of the values of and
, we see that the answer is simply:
Solution 4 (quick)
Notice that if , then
, so there is a bijection between the number of ordered quadruples with
and the number of ordered quadruples with
.
Quick counting gives that the number of ordered quadruples with is 50. To count this, consider our numbers
. Notice that if, for example,
, that the average of
and
must both be
. In this way, there is a symmetry for this case, centered at
. If instead, say,
, an odd number, then there is symmetry with
about
. Further, the number of cases for each of these centers of symmetry correspond to a triangular number. Eg centered at
, there is
case for each and so on, until centered at
, there are
possible cases. Adding these all, we have
.
Thus the answer is
Solution 5
Think about a,b,c,and d as distinct objects, that we must place in 4 of 10 spaces. However, in only 1 of 24 of these combinations, will the placement of these objects satisfy the condition in the problem. So we know the total number of ordered quadruples is
Next, intuitively, the number of quadruples where is equal to the number of quadruples where
. So we need to find the number of quadruples where the two quantities are equal. To do this, all we have to do is consider the cases when
ranges from 3 to 9. It would seem natural that a range of 3 would produce 1 option, and a range of 4 would produce 2 options. However, since b and c cannot be equal, a range of 3 or 4 produces 1 option each, a range of 5 or 6 produces 2 options each, a range of 7 or 8 produces 3 options each, and a range of 9 will produce 4 options. In addition, a range of n has 10-n options for combinations of a and d. Multiplying the number of combinations of a and d by the corresponding number of options for b and c gives us 50 total quadruplets where
.
So the answer will be
Solution 6
Let and
and
for positive integers
In order to satisfy the other condition we need
so we let
Now the only other condition we need to satisfy so
This condition can be transformed into
for positive
Now we use generating functions to finish. We find the generating function of the whole expression is
and we are looking for the
coefficient. This simplifies to finding the
coefficient of
Now this expression simplifies to
The
coefficient ends up to be
Solution 7 (Slightly Slower)
First, let and
. If
, then
can be from
to
. If
, then
to
. If
, then
is between
and
. We find a pattern that whenever
increases by
, when
and
are stationary, then the possible values of
decrease by 2, unless it gets to zero or negative, in which case that case ends. Counting up, we have
different possibilities when
and
. For
and
,
, then
can be from
to
. If
, then
can be from
to
, and so on. Notice that the possible values for each case of
gets one less than if
were one greater, unless that number is zero, in which it stays zero. We then use this pattern to find all the values:
Note
Note that the first value of each of the rows, if we arrange them based on values of , is deleted after each row.
Solution 8
Rearranging the equation obtains . Let
,
,
,
,
. Add up all of these newly defined equations to obtain
. Note that since all
were defined to be
, to form our stars and bars argument we can let
for all
. Then we obtain
where
is nonnegative. Now, we can move the
term to the other side and perform casework.
If : 5 objects for 4 variables ->
If : 3 objects for 4 variables ->
If : 1 object for 4 variables ->
Adding all of these cases up, we get as our requested answer.
~sigma
See also
2011 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 5 |
Followed by Problem 7 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.