Cube roots of a positive real constant from one complex root, roots after multiplying by i, and the area of their triangle

Cube roots of a positive real constant from one complex root, roots after multiplying by i, and the area of their triangle

IB Year 6 | Grade 12
8 marks

Let \(c\) be a positive real constant, and consider the equation \(z^3=c\) for \(z\in\mathbb{C}\).

It is known that \(z=-2+2i\sqrt3\) satisfies this equation.

  1. Find the value of \(c\).[2]
  2. Write down the remaining two roots of \(z^3=c\), each in the form \(a+bi\) with \(a,b\in\mathbb{R}\).[2]
  3. Find every root of the equation \((iz)^3=c\), each in the form \(a+bi\) with \(a,b\in\mathbb{R}\).[2]
  4. On an Argand diagram, the roots found in part (c) are plotted as the points \(A\), \(B\) and \(C\).
    Find the area of triangle \(ABC\).[2]

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Answer:(a) \(64\) (b) \(-2-2i\sqrt3,\ 4\) (c) \(2\sqrt3+2i,\ -2\sqrt3+2i,\ -4i\) (d) \(12\sqrt3\)

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