Complex Cosine Function/Examples/4 cos z = 3+i

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Example of Complex Cosine Function

Let:

$4 \cos z = 3 + i$

Then:

$z = \dfrac {\paren {8 n + 1} \pi} 4 - \dfrac {i \ln 2} 2$ for $n \in \Z$

or:

$z = \dfrac {\paren {8 m - 1} i \pi} 4 + \dfrac {i \ln 2} 2$ for $m \in \Z$


Proof

\(\ds 4 \cos z\) \(=\) \(\ds 3 + i\)
\(\ds \leadsto \ \ \) \(\ds 4 \paren {\dfrac {e^{i z} + e^{-i z} } 2}\) \(=\) \(\ds 3 + i\) Euler's Cosine Identity
\(\ds \leadsto \ \ \) \(\ds 2 e^{2 i z} - \paren {3 + i} e^{i z} + 2\) \(=\) \(\ds 0\) mulitplying by $e^{i z}$ and rearranging
\(\ds \leadsto \ \ \) \(\ds e^{i z}\) \(=\) \(\ds \dfrac {3 + i \pm \sqrt {\paren {3 + i}^2 - 4 \times 2 \times 2} } {2 \times 2}\) Quadratic Formula
\(\ds \) \(=\) \(\ds \dfrac {3 + i \pm \sqrt {9 - 1 + 6 i - 16} } 4\) Square of Sum
\(\ds \) \(=\) \(\ds \dfrac {3 + i \pm \sqrt {-8 + 6 i} } 4\) simplification
\(\ds \) \(=\) \(\ds \dfrac {3 + i \pm \paren {1 + 3 i} } 4\) Square Root of $-8 + 6 i$
\(\ds \) \(=\) \(\ds \dfrac {4 + 4 i} 4 \text { or } \dfrac {2 - 2 i} 4\)
\(\ds \) \(=\) \(\ds 1 + i \text { or } \dfrac 1 2 - \dfrac i 2\)


Thus we have:

\(\ds e^{i z}\) \(=\) \(\ds 1 + i\)
\(\ds \leadsto \ \ \) \(\ds i z\) \(=\) \(\ds \ln \paren {1 + i}\) Definition 2 of Complex Logarithm
\(\ds \) \(=\) \(\ds \ln \paren {\sqrt 2 \exp \paren {\dfrac {i \pi} 4} }\) Definition of Exponential Form of Complex Number
\(\ds \) \(=\) \(\ds \ln \sqrt 2 + \dfrac {i \pi} 4 + 2 n i \pi\) Definition 1 of Complex Logarithm
\(\ds \) \(=\) \(\ds \dfrac {\ln 2} 2 + \dfrac {\paren {8 n + 1} i \pi} 4\) simplification
\(\ds \leadsto \ \ \) \(\ds z\) \(=\) \(\ds -\dfrac {i \ln 2} 2 + \dfrac {\paren {8 n + 1} \pi} 4\) multiplying through by $-i = \dfrac 1 i$


and:

\(\ds e^{i z}\) \(=\) \(\ds \dfrac 1 2 - \dfrac i 2\)
\(\ds \leadsto \ \ \) \(\ds i z\) \(=\) \(\ds \ln \paren {\dfrac 1 2 - \dfrac i 2}\) Definition 2 of Complex Logarithm
\(\ds \) \(=\) \(\ds \ln \paren {\dfrac 2 {\sqrt 2} \exp \paren {-\dfrac \pi 4} }\) Definition of Exponential Form of Complex Number
\(\ds \) \(=\) \(\ds \ln \dfrac 2 {\sqrt 2} + \dfrac {-i \pi} 4 + 2 m i \pi\) Definition 1 of Complex Logarithm
\(\ds \) \(=\) \(\ds -\dfrac {\ln 2} 2 + \dfrac {\paren {8 m - 1} i \pi} 4\) simplification
\(\ds \leadsto \ \ \) \(\ds z\) \(=\) \(\ds \dfrac {i \ln 2} 2 + \dfrac {\paren {8 m - 1} \pi} 4\) multiplying through by $-i = \dfrac 1 i$

$\blacksquare$


Sources

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