Equation relating Points of Parallelogram in Complex Plane/Mistake
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Source Work
1960: Walter Ledermann: Complex Numbers:
- Chapter $2$: Geometrical Representations
- Exercise $7$
This mistake can be seen in the $1960$ edition as published by Routledge & Kegan Paul.
Mistake
- The vertices parallelogram $ABVU$ are represented by the complex numbers $a, b, v, u$ respectively. The angle $UAB$ is equal to $\alpha$ and $\cmod {UA} = \lambda \cmod {AB}$. Prove that $u = \paren {1 - q} a + q b$ and $v = -q a + \paren {1 + q} b$, where $q = \lambda e^{i \alpha}$.
The configuration as described is:
However, because of the sense of $UAB$, this leads to:
- $u = \paren {1 - q} a + q b$ and $v = -q a + \paren {1 + q} b$, where $q = \lambda e^{-i \alpha}$.
The question as it was intended to be stated should contain:
- The angle $BAU$ is equal to $\alpha$ ...
See Equation relating Points of Parallelogram in Complex Plane for a solution to the correctly-stated exercise.
Sources
- 1960: Walter Ledermann: Complex Numbers ... (previous) ... (next): $\S 2$. Geometrical Representations: Exercise $7$