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Algebra Difficulty 6.0 AIME, harder Prove it Turkey

Find all pairs of real numbers (x,y)(x, y) satisfying the following conditions:
x2+y2+x+y=xy(x+y)1027x^2 + y^2 + x + y = xy(x + y) - \frac{10}{27}
xy259.|xy| \le \frac{25}{9}.

Solutions — 2

Solution 1

Answer: (x,y)=(1/3,1/3)(x, y) = (-1/3, -1/3), (5/3,5/3)(5/3, 5/3).
Firstly, we obtain that
x2+y2+x+yxy(x+y)+2=(1x)(1y)(x+y+2) x^2 + y^2 + x + y - xy(x + y) + 2 = -(1-x)(1-y)(x+y+2)
and hence we get
(1x)(1y)(x+y+2)=6427. (1-x)(1-y)(x+y+2) = \frac{64}{27}.
Let k3=1xk^3 = 1-x, 3=1y\ell^3 = 1-y, m3=x+y+2m^3 = x+y+2. Then we have
k3+3+m3=4andkm=43. k^3 + \ell^3 + m^3 = 4 \quad \text{and} \quad k\ell m = \frac{4}{3}.
This shows that k3+3+m3=3kmk^3 + \ell^3 + m^3 = 3k\ell m and it can be expressed as
(k++m)((k)2+(m)2+(mk)2)=0. (k + \ell + m)((k - \ell)^2 + (\ell - m)^2 + (m - k)^2) = 0.
The last equality implies that either k++m=0k + \ell + m = 0 or k==mk = \ell = m.
For k==mk = \ell = m, we obtain the solution x=y=1/3x = y = -1/3.
Let k++m=0k + \ell + m = 0. In this case using km=4/3k\ell m = 4/3, we get k(k+)=4/3k\ell(k + \ell) = -4/3. Consider the condition xy=(1k3)(13)25/9|xy| = |(1-k^3)(1-\ell^3)| \le 25/9. Define u=k+u = k + \ell, v=kv = k\ell. In this case, we get
uv=43and(1k3)(13)=u3v3+3259. uv = -\frac{4}{3} \quad \text{and} \quad |(1-k^3)(1-\ell^3)| = |u^3 - v^3 + 3| \le \frac{25}{9}.
These two conditions imply that
529u3v3=u3+6427u329() -\frac{52}{9} \le u^3 - v^3 = u^3 + \frac{64}{27u^3} \le -\frac{2}{9} \quad (*)
By AM-GM, we get u24v=16/(3u)u^2 \ge 4v = -16/(3u) which is equivalent to u>0u > 0 or u316/3u^3 \le -16/3. From (*), we conclude that u<0u < 0 and hence u316/3u^3 \le -16/3. In this case since u3+16/30u^3 + 16/3 \le 0 and 4/(9u3)+111/12>04/(9u^3) + 1 \ge 11/12 > 0 we get that
u3+6427u3+529=(u3+163)(49u3+1)0 u^3 + \frac{64}{27u^3} + \frac{52}{9} = \left(u^3 + \frac{16}{3}\right) \left(\frac{4}{9u^3} + 1\right) \le 0
Therefore, (*) holds only if u3=16/3u^3 = -16/3 and u2=4vu^2 = 4v which implies that k==23/3k = \ell = -\sqrt[3]{2}/3. This yields the solution x=y=5/3x = y = 5/3. Both obtained solutions satisfy the problem conditions.

Solution 2

Let x+y=ax+y=a, xy=bxy=b. Then by AM-GM, we have a24ba^2 \ge 4b and hence
x2+y2+x+yxy(x+y)=a2+ab(a+2)=1027, x^2 + y^2 + x + y - xy(x + y) = a^2 + a - b(a + 2) = -\frac{10}{27},
b=a2+a+1027a+2a24, b = \frac{a^2 + a + \frac{10}{27}}{a + 2} \le \frac{a^2}{4},
b=a2+a+1027a+2259. |b| = \left| \frac{a^2 + a + \frac{10}{27}}{a + 2} \right| \le \frac{25}{9}.
Therefore, we get
a24a2+a+1027a+2=(a+23)2(a103)4(a+2)0(1), \frac{a^2}{4} - \frac{a^2 + a + \frac{10}{27}}{a + 2} = \frac{\left(a + \frac{2}{3}\right)^2 \left(a - \frac{10}{3}\right)}{4(a + 2)} \ge 0 \quad (1),
a2+a+1027a+2259=(a103)(a+149)a+20(2), \frac{a^2 + a + \frac{10}{27}}{a + 2} - \frac{25}{9} = \frac{\left(a - \frac{10}{3}\right)\left(a + \frac{14}{9}\right)}{a + 2} \le 0 \quad (2),
a2+a+1027a+2+259=(a+179)2+19181a+20(3). \frac{a^2 + a + \frac{10}{27}}{a + 2} + \frac{25}{9} = \frac{\left(a + \frac{17}{9}\right)^2 + \frac{191}{81}}{a + 2} \ge 0 \quad (3).
Using (1), (2) and (3), we obtain the following solutions:
(1)    a<2ora103ora=23, (1) \iff a < -2 \quad \text{or} \quad a \ge \frac{10}{3} \quad \text{or} \quad a = -\frac{2}{3},
(2)    a<2or149a103, (2) \iff a < -2 \quad \text{or} \quad -\frac{14}{9} \le a \le \frac{10}{3},
(3)    a>2. (3) \iff a > -2.
Hence, the only solutions are a=23a = -\frac{2}{3} and a=103a = \frac{10}{3}. In both cases, a2=4ba^2 = 4b and it follows that x=y=a2x = y = \frac{a}{2}. The obtained solutions (x,y)=(1/3,1/3)(x, y) = (-1/3, -1/3), (5/3,5/3)(5/3, 5/3) satisfy the problem conditions.

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