Maths Olympiad Prep

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Geometry Difficulty 7.2 National olympiad, round 2 Prove it

Let aa, bb and cc be lengths of sides of triangle ABCABC. Prove that at least one of the equations x22bx+2ac=0x^2-2bx+2ac=0 x22cx+2ab=0x^2-2cx+2ab=0 x22ax+2bc=0x^2-2ax+2bc=0 does not have real solutions

Solution

1. Assume that each of the equations has at least one real solution.

For a quadratic equation ax2+bx+c=0 ax^2 + bx + c = 0 to have real solutions, its discriminant must be nonnegative. The discriminant Δ\Delta of the quadratic equation x22bx+2ac=0 x^2 - 2bx + 2ac = 0 is given by:
Δ=(2b)2412ac=4b28ac \Delta = (-2b)^2 - 4 \cdot 1 \cdot 2ac = 4b^2 - 8ac
For this equation to have real solutions, we must have:
4b28ac0    b22ac 4b^2 - 8ac \geq 0 \implies b^2 \geq 2ac

2. Apply the same reasoning to the other two equations:

For the equation x22cx+2ab=0 x^2 - 2cx + 2ab = 0 , the discriminant is:
Δ=(2c)2412ab=4c28ab \Delta = (-2c)^2 - 4 \cdot 1 \cdot 2ab = 4c^2 - 8ab
For this equation to have real solutions, we must have:
4c28ab0    c22ab 4c^2 - 8ab \geq 0 \implies c^2 \geq 2ab

For the equation x22ax+2bc=0 x^2 - 2ax + 2bc = 0 , the discriminant is:
Δ=(2a)2412bc=4a28bc \Delta = (-2a)^2 - 4 \cdot 1 \cdot 2bc = 4a^2 - 8bc
For this equation to have real solutions, we must have:
4a28bc0    a22bc 4a^2 - 8bc \geq 0 \implies a^2 \geq 2bc

3. Add the inequalities obtained:

b22ac b^2 \geq 2ac
c22ab c^2 \geq 2ab
a22bc a^2 \geq 2bc

Adding these three inequalities, we get:
a2+b2+c22ac+2ab+2bc a^2 + b^2 + c^2 \geq 2ac + 2ab + 2bc

4. Compare with the triangle inequality:

The triangle inequality states that the sum of the lengths of any two sides of a triangle must be greater than the length of the remaining side. This implies:
a+b>c,b+c>a,c+a>b a + b > c, \quad b + c > a, \quad c + a > b
Squaring these inequalities and adding them, we get:
(a+b)2+(b+c)2+(c+a)2>c2+a2+b2 (a + b)^2 + (b + c)^2 + (c + a)^2 > c^2 + a^2 + b^2
Simplifying, we get:
2(a2+b2+c2+ab+bc+ca)>a2+b2+c2 2(a^2 + b^2 + c^2 + ab + bc + ca) > a^2 + b^2 + c^2
a2+b2+c2<2(ab+bc+ca) a^2 + b^2 + c^2 < 2(ab + bc + ca)

5. Derive the contradiction:

From the inequalities derived from the discriminants, we have:
a2+b2+c22(ab+bc+ca) a^2 + b^2 + c^2 \geq 2(ab + bc + ca)
But from the triangle inequality, we have:
a2+b2+c2<2(ab+bc+ca) a^2 + b^2 + c^2 < 2(ab + bc + ca)
This is a contradiction. Therefore, our initial assumption that each of the equations has at least one real solution must be false.

\blacksquare

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.