Obligatory During-MathZoom Post
by djmathman, Jul 6, 2012, 9:34 PM
Darn Dr. Wang's worksheets are pretty hard.
Solution
Solution
Example Algebra Problem wrote:
Find the real solutions: ![\[\begin{cases}x^3+y^3+x^3y^3=17\\x+y+xy=5\end{cases}\]](//latex.artofproblemsolving.com/8/f/7/8f7fd4d9d7389517c479c3658791db6402120a6d.png)
![\[\begin{cases}x^3+y^3+x^3y^3=17\\x+y+xy=5\end{cases}\]](http://latex.artofproblemsolving.com/8/f/7/8f7fd4d9d7389517c479c3658791db6402120a6d.png)
Solution
Let
and
. The second equation transforms to
. The first equation transforms to ![\[\left[(x+y)^3-3xy(x+y)\right]+x^3y^3=a^3-3ab+b^3=17.\]](//latex.artofproblemsolving.com/3/7/c/37c2b18869687d05f660ea3b7d88331c2459f305.png)
Next, noting the expansion for the cube of a sum gives us![\[(a+b)^3=a^3+b^3+3ab(a+b).\]](//latex.artofproblemsolving.com/5/d/5/5d5329db6eb16a367ab46fe5c36053d46126d970.png)
Since we know
, this simplifies to ![\[a^3+15ab+b^3=125.\]](//latex.artofproblemsolving.com/8/0/9/809d24bd3d99f682c971376fdc7f4e4db5229cf2.png)
Now we have the system of equations![\[\begin{cases}a^3-3ab+b^3=17\\a^3+15ab+b^3=125\end{cases}\]](//latex.artofproblemsolving.com/2/f/6/2f62c7db3930c950a5d5e7d7b85d55b8c5c41324.png)
Subtracting the second one from the first one gives us
. Since
and
,
and
are some permutation of
and
. If
and
, then
so
or
. If
and
, then
so
and
are the roots of the equation
, which has a negative descriminant. Therefore, the only solutions are
.



![\[\left[(x+y)^3-3xy(x+y)\right]+x^3y^3=a^3-3ab+b^3=17.\]](http://latex.artofproblemsolving.com/3/7/c/37c2b18869687d05f660ea3b7d88331c2459f305.png)
Next, noting the expansion for the cube of a sum gives us
![\[(a+b)^3=a^3+b^3+3ab(a+b).\]](http://latex.artofproblemsolving.com/5/d/5/5d5329db6eb16a367ab46fe5c36053d46126d970.png)
Since we know

![\[a^3+15ab+b^3=125.\]](http://latex.artofproblemsolving.com/8/0/9/809d24bd3d99f682c971376fdc7f4e4db5229cf2.png)
Now we have the system of equations
![\[\begin{cases}a^3-3ab+b^3=17\\a^3+15ab+b^3=125\end{cases}\]](http://latex.artofproblemsolving.com/2/f/6/2f62c7db3930c950a5d5e7d7b85d55b8c5c41324.png)
Subtracting the second one from the first one gives us









![\[\begin{cases}x+y=2\\xy=3\end{cases}\]](http://latex.artofproblemsolving.com/e/8/c/e8cf13f4a589121bf9c0ae5282a0509b80967118.png)




![\[\begin{cases}x+y=3\\xy=2\end{cases}\]](http://latex.artofproblemsolving.com/3/2/9/329c64462a0009783d5c57bad4834a073762511e.png)




1988 AIME #13 wrote:
Find
if
and
are integers such that
is a factor of
.





Let
be real numbers such that
![\[ax^{17}+bx^{16}+1=(x^2-x-1)(a_{15}x^{15}+a_{14}x^{14}+a_{13}x^{13}+\cdots+a_2x^2+a_1x-1).\]](//latex.artofproblemsolving.com/d/0/9/d094557ee777192ac447a3a4519904a5b5d83e69.png)
Now, we determine the coefficients
through
one by one. The sum of the
coefficients is
, so we have
, which means
. The sum of the
coefficients is also
, so we can say that
which after plugging in
gives
. By repeating this same pattern, we can determine that
,
,
... these seem like alternating signs of the Fibonacchi sequence. After proving this, we compute the first
fibonacchi numbers besides the first
) to find
, which we know is positive since it is the coefficient of a term with odd degree: ![\[1,2,3,5,8,13,21,34,55,89,144233,377,610,\boxed{987}.\]](//latex.artofproblemsolving.com/4/0/9/4097fa2480126bb1b146c501a95b2ba52bdf5191.png)
Since the coefficient of
is the same as the coefficient of
, we are done.

![\[ax^{17}+bx^{16}+1=(x^2-x-1)(a_{15}x^{15}+a_{14}x^{14}+a_{13}x^{13}+\cdots+a_2x^2+a_1x-1).\]](http://latex.artofproblemsolving.com/d/0/9/d094557ee777192ac447a3a4519904a5b5d83e69.png)
Now, we determine the coefficients








![\[1^2(-1)+a_1(-1)-a_2=0,\]](http://latex.artofproblemsolving.com/5/6/a/56a72df2ac3e7a392be2ec3d79de12a667e5dfb5.png)








![\[1,2,3,5,8,13,21,34,55,89,144233,377,610,\boxed{987}.\]](http://latex.artofproblemsolving.com/4/0/9/4097fa2480126bb1b146c501a95b2ba52bdf5191.png)
Since the coefficient of

