2022 AMC 10A Problems/Problem 20
Contents
Problem
A four-term sequence is formed by adding each term of a four-term arithmetic sequence of positive integers to the corresponding term of a four-term geometric sequence of positive integers. The first three terms of the resulting four-term sequence are ,
, and
. What is the fourth term of this sequence?
Solution 1
Let the arithmetic sequence be and the geometric sequence be
We are given that
and we wish to find
Subtracting the first equation from the second and the second equation from the third, we get
Subtract these results, we get
Note that either or
We proceed with casework:
- If
then
and
The arithmetic sequence is
arriving at a contradiction.
- If
then
and
The arithmetic sequence is
and the geometric sequence is
This case is valid.
Therefore, The answer is
~mathboy282 ~MRENTHUSIASM
Solution 2 (Elimination)
Start similarly to Solution 1 and deduce the three equations
Then, add the last two equations and take away the first equation to get
We can solve for this in terms of what we want:
.
We're looking for . We can substitute our value of
in here to get
Since our sequence only has positive integers we can now check by the answer choices. For each answer choice, we can subtract
and factor it to see if it has a perfect square factor and at least one other factor and those should differ by
.
From this, the only possible answer choices are
and
, where
. To solve for
, we look back to the given equations above.
We are looking for . If
were the answer, then we know that
would have to be divisible by
and
would equal
. Looking at our second equation, if this were the case, then
would also have to be divisible by
. However, this contradicts the third equation, as all variables are divisible by
, but their sum isn't. So,
is our answer.
Video Solution by OmegaLearn
~ pi_is_3.14
See Also
2022 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 19 |
Followed by Problem 21 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
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