GeometryDifficulty 7.0National olympiadFind the answer
For some complex number ω with ∣ω∣=2016, there is some real λ>1 such that ω,ω2, and λω form an equilateral triangle in the complex plane. Then, λ can be written in the form ca+b, where a,b, and c are positive integers and b is squarefree. Compute a+b+c.
A number or a short expression. Spacing and $ signs are ignored.
Solution
1. Let A be the point representing ω in the complex plane, B represent ω2, and C be λω. Let O be the origin (0) in the complex plane. We wish to find λ such that △ABC is equilateral.
2. The side length AC has length 2016(λ−1) and OA has length 2016. Notice that if △ABC is equilateral, then ∠OAB is 120∘. Also, OB has length 20162.
3. Applying the law of cosines to △OAB, we get: AB2=OA2+OB2−2⋅OA⋅OB⋅cos(120∘) Substituting the known values: (2016(λ−1))2=20162+(20162)2−2⋅2016⋅20162⋅cos(120∘) Since cos(120∘)=−21, the equation becomes: 20162(λ−1)2=20162+20164+20162(λ−1)
4. Simplifying the equation: (λ−1)2+1+(λ−1)=20162 λ2−λ+1−20162=0
5. Solving the quadratic equation λ2−λ+1−20162=0 using the quadratic formula λ=2a−b±b2−4ac: λ=21±1−4(1−20162) λ=21±1+4⋅20162−4 λ=21±1+4⋅20162−4 λ=21±4⋅20162−3 λ=21±4⋅20162−3
6. Since λ>1, we take the positive root: λ=21+4⋅20162−3
7. We need to express λ in the form ca+b. Here, a=1, b=4⋅20162−3, and c=2.