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Algebra Difficulty 6.3 National Olympiad Prove it Croatia

Determine all real numbers aa such that the equation
x2(5a)x+a211a46=0 x^2 - (5-a)x + a^2 - 11a - 46 = 0
has two real solutions, one of which is less than 22, and the other greater than 22.

Solution

Let the roots of the quadratic be x1x_1 and x2x_2, with x1<2<x2x_1 < 2 < x_2.

The quadratic equation is x2(5a)x+a211a46=0x^2 - (5-a)x + a^2 - 11a - 46 = 0.

By Vieta's formulas:
- x1+x2=5ax_1 + x_2 = 5 - a
- x1x2=a211a46x_1 x_2 = a^2 - 11a - 46

Since the equation must have two real solutions, the discriminant must be non-negative:
D = [-(5-a)] 2 - 4(a 2 - 11a - 46) = (5-a) 2 - 4(a 2 - 11a - 46)\text{D = [-(5-a)] 2 - 4(a 2 - 11a - 46) = (5-a) 2 - 4(a 2 - 11a - 46)}
Expand:
(5a)2=2510a+a2 (5-a)^2 = 25 - 10a + a^2
So,
D=2510a+a24(a211a46) D = 25 - 10a + a^2 - 4(a^2 - 11a - 46)
=2510a+a24a2+44a+184 = 25 - 10a + a^2 - 4a^2 + 44a + 184
=(25+184)+(10a+44a)+(a24a2) = (25 + 184) + (-10a + 44a) + (a^2 - 4a^2)
=209+34a3a2 = 209 + 34a - 3a^2
So, D0D \geq 0:
209+34a3a20 209 + 34a - 3a^2 \geq 0

Now, the roots must be on opposite sides of 22.
For a quadratic ax2+bx+c=0ax^2 + bx + c = 0, the value at x=2x = 2 is:
f(2)=(2)2(5a)(2)+a211a46 f(2) = (2)^2 - (5-a)(2) + a^2 - 11a - 46
=42(5a)+a211a46 = 4 - 2(5-a) + a^2 - 11a - 46
=410+2a+a211a46 = 4 - 10 + 2a + a^2 - 11a - 46
=(41046)+(2a11a)+a2 = (4 - 10 - 46) + (2a - 11a) + a^2
=(52)9a+a2 = (-52) - 9a + a^2
=a29a52 = a^2 - 9a - 52

For the roots to be on opposite sides of 22, f(2)f(2) must be negative (since the parabola opens upwards):
f(2)<0    a29a52<0 f(2) < 0 \implies a^2 - 9a - 52 < 0

So, we have two inequalities:
1. 209+34a3a20209 + 34a - 3a^2 \geq 0
2. a29a52<0a^2 - 9a - 52 < 0

Let's solve each.

First inequality:
209+34a3a20 209 + 34a - 3a^2 \geq 0
3a2+34a+2090 -3a^2 + 34a + 209 \geq 0
3a234a2090 3a^2 - 34a - 209 \leq 0
Find the roots:
a=34±(34)243(209)23 a = \frac{34 \pm \sqrt{(-34)^2 - 4 \cdot 3 \cdot (-209)}}{2 \cdot 3}
=34±1156+25086 = \frac{34 \pm \sqrt{1156 + 2508}}{6}
=34±36646 = \frac{34 \pm \sqrt{3664}}{6}
366460.5 \sqrt{3664} \approx 60.5
So,
a1=34+60.5694.5615.75 a_1 = \frac{34 + 60.5}{6} \approx \frac{94.5}{6} \approx 15.75
a2=3460.5626.564.42 a_2 = \frac{34 - 60.5}{6} \approx \frac{-26.5}{6} \approx -4.42
So, 3a234a20903a^2 - 34a - 209 \leq 0 for 4.42a15.75-4.42 \leq a \leq 15.75.

Second inequality:
a29a52<0 a^2 - 9a - 52 < 0
Find the roots:
a=9±81+2082 a = \frac{9 \pm \sqrt{81 + 208}}{2}
=9±2892 = \frac{9 \pm \sqrt{289}}{2}
=9±172 = \frac{9 \pm 17}{2}
So,
a1=9+172=13 a_1 = \frac{9 + 17}{2} = 13
a_2 = 9\text{9} - 17}{2} = -4

So, a29a52<0a^2 - 9a - 52 < 0 for 4<a<13-4 < a < 13.

The intersection of the two intervals:
4.42a15.75-4.42 \leq a \leq 15.75 and 4<a<13-4 < a < 13

So, aa must satisfy 4<a<13-4 < a < 13.

Final answer:
All real numbers aa such that 4<a<13-4 < a < 13.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.