Difference between revisions of "2018 AIME II Problems/Problem 8"
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==Solution 2 (Case Bashing)== | ==Solution 2 (Case Bashing)== | ||
− | We'll refer to the moves <math>(x + 1, y)</math>, <math>(x + 2, y)</math>, <math>(x, y + 1)</math>, and <math>(x, y + 2)</math> as <math>R_1</math>, <math>R_2</math>, <math>U_1</math>, and <math>U_2</math>, respectively. Then the possible | + | We'll refer to the moves <math>(x + 1, y)</math>, <math>(x + 2, y)</math>, <math>(x, y + 1)</math>, and <math>(x, y + 2)</math> as <math>R_1</math>, <math>R_2</math>, <math>U_1</math>, and <math>U_2</math>, respectively. Then the possible sequences of moves that will take the frog from <math>(0,0)</math> to <math>(4,4)</math> are all the permutations of <math>U_1U_1U_1U_1R_1R_1R_1R_1</math>, <math>U_2U_1U_1R_1R_1R_1R_1</math>, <math>U_1U_1U_1U_1R_2R_1R_1</math>, <math>U_2U_1U_1R_2R_1R_1</math>, <math>U_2U_2R_1R_1R_1R_1</math>, <math>U_1U_1U_1U_1R_2R_2</math>, <math>U_2U_2R_2R_1R_1</math>, <math>U_2U_1U_1R_2R_2</math>, and <math>U_2U_2R_2R_2</math>. We can reduce the number of cases using symmetry. |
Case 1: <math>U_1U_1U_1U_1R_1R_1R_1R_1</math> | Case 1: <math>U_1U_1U_1U_1R_1R_1R_1R_1</math> |
Revision as of 18:28, 25 March 2018
Problem
A frog is positioned at the origin of the coordinate plane. From the point , the frog can jump to any of the points
,
,
, or
. Find the number of distinct sequences of jumps in which the frog begins at
and ends at
.
Solution 1
We solve this problem by working backwards. Notice, the only points the frog can be on to jump to in one move are
and
. This applies to any other point, thus we can work our way from
to
, recording down the number of ways to get to each point recursively.
,
,
,
Solution 2 (Case Bashing)
We'll refer to the moves ,
,
, and
as
,
,
, and
, respectively. Then the possible sequences of moves that will take the frog from
to
are all the permutations of
,
,
,
,
,
,
,
, and
. We can reduce the number of cases using symmetry.
Case 1:
There are possibilities for this case.
Case 2: or
There are possibilities for this case.
Case 3:
There are possibilities for this case.
Case 4: or
There are possibilities for this case.
Case 5: or
There are possibilities for this case.
Case 6:
There are possibilities for this case.
Adding up all these cases gives us ways.
2018 AIME II (Problems • Answer Key • Resources) | ||
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Followed by Problem 9 | |
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