# Thread: Solve cos z = 5

1. ## Solve cos z = 5

HOW DO I DO THIS?!

Solve $\displaystyle cos(z) = 5$

need someone to go through slowly with me lol :S

2. ## Re: Solve cos z = 5

You should note that $\displaystyle \displaystyle \cos{(z)} = \cos{(x + iy)} = \cos{(x)}\cosh{(y)} - i\sin{(x)}\sinh{(y)}$, so

$\displaystyle \displaystyle \cos{(x)}\cosh{(y)} = 5$ and $\displaystyle \displaystyle -\sin{(x)}\sinh{(y)} = 0$.

From the second equation, we can see that either $\displaystyle \displaystyle \sin{(x)} = 0 \implies x = n\pi, n \in \mathbf{Z}$ or $\displaystyle \displaystyle \sinh{(y)} = 0 \implies y = 0$.

If $\displaystyle \displaystyle x = n\pi$ then

\displaystyle \displaystyle \begin{align*} \cos{(x)}\cosh{(y)} &= 5 \\ \cos{(n\pi)}\cosh{(y)} &= 5 \\ (-1)^n\cosh{(y)} &= 5 \\ \cosh{(y)} &= \frac{5}{(-1)^n} \end{align*}

and since the hyperbolic cosine is always positive, that means we can only accept even values for $\displaystyle \displaystyle n$, so $\displaystyle \displaystyle n = 2m, m\in \mathbf{Z}$ and $\displaystyle \displaystyle y = \textrm{arcosh}\,{(5)}$.

So the first solution is $\displaystyle \displaystyle z = 2m\pi + \textrm{arcosh}\,{(5)}, m \in \mathbf{Z}$.

See if you can get the second solution, for where $\displaystyle \displaystyle y = 0$.

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# general solution of cosz=.5

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