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2025 AMC 12A Problem 17

Problem 17 of 25IntermediateAlgebraGeometry

The polynomial (z+i)(z+2i)(z+3i)+10(z + i)(z + 2i)(z + 3i) + 10 has three roots in the complex plane, where i=1.i = \sqrt{-1}. What is the area of the triangle formed by these roots?

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Solution

The sum of the roots is 6i,-6i, so the centroid is 2i.-2i. Substituting z=u2i,z = u - 2i, (ui)(u)(u+i)+10=u(u2+1)+10=u3+u+10. \begin{gathered} (u - i)(u)(u + i) + 10 \\ = u(u^2 + 1) + 10 \\ = u^3 + u + 10. \end{gathered} Since u=2u = -2 is a root, u3+u+10u^3 + u + 10 =(u+2)(u22u+5),= (u + 2)(u^2 - 2u + 5), giving roots u=2u = -2 and u=1±2i.u = 1 \pm 2i. These are the points (2,0),(-2, 0), (1,2),(1, 2), (1,2).(1, -2). The base between (1,2)(1, 2) and (1,2)(1, -2) has length 4,4, at horizontal distance 33 from (2,0),(-2, 0), so the area is 12(4)(3)=6.\dfrac{1}{2}(4)(3) = 6. Translation does not change the area. Thus, the correct answer is A.

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Concepts: complex number · polynomial · triangle area

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