2020 AMC 10B Problems/Problem 9
- The following problem is from both the 2020 AMC 10B #9 and 2020 AMC 12B #8, so both problems redirect to this page.
Contents
Problem
How many ordered pairs of integers satisfy the equation
Solutions
Solution 1
Rearranging the terms and and completing the square for yields the result
. Then, notice that
can only be
,
and
because any value of
that is greater than 1 will cause the term
to be less than
, which is impossible as
must be real. Therefore, plugging in the above values for
gives the ordered pairs
,
,
, and
gives a total of
ordered pairs.
Solution 2
Bringing all of the terms to the LHS, we see a quadratic equation in terms of
. Applying the quadratic formula, we get
In order for
to be real, which it must be given the stipulation that we are seeking integral answers, we know that the discriminant,
must be nonnegative. Therefore,
Here, we see that we must split the inequality into a compound, resulting in
.
The only integers that satisfy this are . Plugging these values back into the quadratic equation, we see that
both produce a discriminant of
, meaning that there is only 1 solution for
. If
, then the discriminant is nonzero, therefore resulting in two solutions for
.
Thus, the answer is .
~Tiblis
Solution 3: Solve for x first
Set it up as a quadratic in terms of y:
Then the discriminant is
This will clearly only yield real solutions when
, because it is always positive.
Then
. Checking each one:
and
are the same when raised to the 2020th power:
This has only has solutions
, so
are solutions.
Next, if
:
Which has 2 solutions, so
and
.
These are the only 4 solutions, so
~edits by BakedPotato66
Solution 4: Solve for y first
Move the term to the other side to get
.
Because for all
, then
.
If or
, the right side is
and therefore
.
When , the right side become
, therefore
.
Our solutions are ,
,
,
. These are the only
solutions, so the answer is
- wwt7535
~ edits by BakedPotato66
Video Solution 1
~IceMatrix
Video Solution 2
~savannahsolver
Video Solution 3
https://youtu.be/zfChnbMGLVQ?t=4251
~ pi_is_3.14
See Also
2020 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 8 |
Followed by Problem 10 | |
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 |
2020 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 7 |
Followed by Problem 9 |
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 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.