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Geometry Difficulty 7.0 National olympiad Find the answer

For some complex number ω\omega with ω=2016|\omega| = 2016, there is some real λ>1\lambda>1 such that ω,ω2,\omega, \omega^{2}, and λω\lambda \omega form an equilateral triangle in the complex plane. Then, λ\lambda can be written in the form a+bc\tfrac{a + \sqrt{b}}{c}, where a,b,a,b, and cc are positive integers and bb is squarefree. Compute a+b+c\sqrt{a+b+c}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Let A A be the point representing ω \omega in the complex plane, B B represent ω2 \omega^2 , and C C be λω \lambda \omega . Let O O be the origin (0) in the complex plane. We wish to find λ \lambda such that ABC \triangle ABC is equilateral.

2. The side length AC AC has length 2016(λ1) 2016(\lambda - 1) and OA OA has length 2016. Notice that if ABC \triangle ABC is equilateral, then OAB \angle OAB is 120 120^\circ . Also, OB OB has length 20162 2016^2 .

3. Applying the law of cosines to OAB \triangle OAB , we get:
AB2=OA2+OB22OAOBcos(120) AB^2 = OA^2 + OB^2 - 2 \cdot OA \cdot OB \cdot \cos(120^\circ)
Substituting the known values:
(2016(λ1))2=20162+(20162)22201620162cos(120) (2016(\lambda - 1))^2 = 2016^2 + (2016^2)^2 - 2 \cdot 2016 \cdot 2016^2 \cdot \cos(120^\circ)
Since cos(120)=12 \cos(120^\circ) = -\frac{1}{2} , the equation becomes:
20162(λ1)2=20162+20164+20162(λ1) 2016^2(\lambda - 1)^2 = 2016^2 + 2016^4 + 2016^2(\lambda - 1)

4. Simplifying the equation:
(λ1)2+1+(λ1)=20162 (\lambda - 1)^2 + 1 + (\lambda - 1) = 2016^2
λ2λ+120162=0 \lambda^2 - \lambda + 1 - 2016^2 = 0

5. Solving the quadratic equation λ2λ+120162=0 \lambda^2 - \lambda + 1 - 2016^2 = 0 using the quadratic formula λ=b±b24ac2a \lambda = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} :
λ=1±14(120162)2 \lambda = \frac{1 \pm \sqrt{1 - 4(1 - 2016^2)}}{2}
λ=1±1+42016242 \lambda = \frac{1 \pm \sqrt{1 + 4 \cdot 2016^2 - 4}}{2}
λ=1±1+42016242 \lambda = \frac{1 \pm \sqrt{1 + 4 \cdot 2016^2 - 4}}{2}
λ=1±42016232 \lambda = \frac{1 \pm \sqrt{4 \cdot 2016^2 - 3}}{2}
λ=1±42016232 \lambda = \frac{1 \pm \sqrt{4 \cdot 2016^2 - 3}}{2}

6. Since λ>1 \lambda > 1 , we take the positive root:
λ=1+42016232 \lambda = \frac{1 + \sqrt{4 \cdot 2016^2 - 3}}{2}

7. We need to express λ \lambda in the form a+bc \frac{a + \sqrt{b}}{c} . Here, a=1 a = 1 , b=4201623 b = 4 \cdot 2016^2 - 3 , and c=2 c = 2 .

8. Compute a+b+c \sqrt{a + b + c} :
a+b+c=1+(4201623)+2=420162 a + b + c = 1 + (4 \cdot 2016^2 - 3) + 2 = 4 \cdot 2016^2
a+b+c=420162=22016=4032 \sqrt{a + b + c} = \sqrt{4 \cdot 2016^2} = 2 \cdot 2016 = 4032

The final answer is 4032 \boxed{4032} .

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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.