Real Number Multiplied by Complex Number
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Theorem
Let $a \in \R$ be a real number.
Let $c + d i \in \C$ be a complex number.
Then:
- $a \times \paren {c + d i} = \paren {c + d i} \times a = a c + i a d$
Proof
$a$ can be expressed as a wholly real complex number $a + 0 i$.
Then we have:
\(\ds a \times \paren {c + d i}\) | \(=\) | \(\ds \paren {a + 0 i} \times \paren {c + d i}\) | Definition of Wholly Real | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {a c - 0 d} + \paren {a d + 0 c} i\) | Definition of Complex Multiplication | |||||||||||
\(\ds \) | \(=\) | \(\ds a c + i a d\) | simplification |
The result for $\paren {c + d i} \times a$ follows from Complex Multiplication is Commutative.
$\blacksquare$
Sources
- 1960: Walter Ledermann: Complex Numbers ... (previous) ... (next): $\S 1.2$. The Algebraic Theory