2015 AMC 10A Problems/Problem 16
Contents
Problem
If , and
, what is the value of
?
Solutions
Solution 1
Note that we can add the two equations to yield the equation
Moving terms gives the equation
We can also subtract the two equations to yield the equation
Moving terms gives the equation
Because we can divide both sides of the equation by
to yield the equation
Substituting this into the equation for that we derived earlier gives
Solution 2 (Algebraic)
Subtract from the left hand side of both equations, and use difference of squares to yield the equations
and
.
It may save some time to find two solutions, and
, at this point. However,
in these solutions.
Substitute into
.
This gives the equation
which can be simplified to
.
Knowing and
are solutions is now helpful, as you divide both sides by
. This can also be done using polynomial division to find
as a factor. This gives
.
Because the two equations and
are symmetric, the
and
values are the roots of the equation, which are
and
.
Squaring these and adding them together gives
.
Solution 3
By graphing the two equations on a piece of graph paper, we can see that the point where they intersect that is not on the line is close to the point
(or
).
, and the closest answer choice to
is
.
Note for solution 3:
This is risky, as could be a viable answer too. Do not use this method unless you're sure about the answer. In other words, this solution is less reliable than the others, so only use it if you can't do the other methods.
Solution 4 (When you can't algebraically manipulate)
This, by the way, is a really cheap way of solving the problem but as long as you get an answer, it doesn't matter.
Looking at the first given equation, begin searching for solutions. Notice how works but when plugged into the
second equation, you get . Now, if you decrease the
value by
to obtain
, plugging it into
the second equation will yield . Now, notice how the LHS is now less than the RHS as opposed to what it had
been when we plugged into the second given equation. We then conclude that there must be a solution between
and
, so calculating
of
and
we obtain
and
Thus we know the answer to be .
This is a really bad solution, but it works. :/
Solution 5 (Very in Depth) Note: In this solution, I explain my entire thought process and the reason why I did each step, so you can read a more intuitive solution rather than a contrived one. It also isn't in Latex, sorry about that, but it shouldn't really affect anything
We first notice that each equation has a 4, which inspires us to isolate the 4 on both equations and equate them to see if we can gather any useful information on x and y. Doing this we get (x-2)^2-y = (y-2)^2 x. Now you might be tempted to expand, but notice we have to perfect squares in our equation: (x-2)^2 and (y-2)^2, which motivates us to take advantage of difference of squares. For simplicity we suppose that (x-2) = a and (y-2) =b; thus our equation is now a^2-y = b^2-x, we manipulate this equation by getting the LHS in a form that lets us do difference of equations, so we rewrite as a^2-b^2 = -(x-y). Now we can finally use our difference of squares to get (a-b)(a+b) = -(x-y). Lets now substitute a and b back to get information solely for x and y (our substitution was to make manipulation easier for us, it isn't necessary though). Doing this we get, (x-y)(x+y-4) = -(x-y). Now let us divide by (x-y) on both sides and clean up the equation; when we do this, we get that x+y = 3. Okay so what may we do from here? Well we got x+y = 3, which means that this is fact is only of use if we can obtain it in another way, so we add the LHS and the RHS of both equations and equate them to make our fact of use. Doing that, we get (x+y +8) = (x-2)^2+ (y-2)^2. Also, when we set this equation up we note only make our fact useful but we also can see that after expanding the RHS we get x^2+y^2 on the RHS too, which makes even a better move that we added both equations because remember our goal is to find x^2+y^2. Expanding and plugging in x+y = 3 we get 11 = x^2+y^2-4(x+y)+8. Oh wait but x+y=3, so we can plug that in and we get x^2+y^2 = 15 or B after cleaning up some of the algebra. ~triggod
Video Solution
https://youtu.be/0W3VmFp55cM?t=15
~ pi_is_3.14
Video Solution
~savannahsolver
See Also
2015 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 15 |
Followed by Problem 17 | |
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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