# Trigonomentic mapping question

• June 14th 2011, 05:51 AM
Glitch
Trigonomentic mapping question
The question
For the mapping f(z) = sinh(z), find and sketch the image of Im(z) = d.

If I'm not mistaken, this is just a horizontal line in the z-plane through some constant d. With mapping questions involving Z, I usually try and write f(z) in terms of z, then substitute it into the equation.

However this one has me stumped. I tried:
Let f(z) = w
$w = \frac{e^z - e^{-z}}{2}$
$2w = e^z - e^{-z}$

Now I'm unsure of how to write this in terms of z. Is this the correct approach? If so, how do I progress?

Thank you.
• June 14th 2011, 06:07 AM
Prove It
Re: Trigonomentic mapping question
$\displaystyle \sinh{(z)} = \sinh{(x + iy)} = \sinh{(x)}\cos{(y)} + i\cosh{(x)}\sin{(y)}$.

You need to plot the imaginary part of this equal to $\displaystyle d$, so

\displaystyle \begin{align*} \cosh{(x)}\sin{(y)} &= d \\ \sin{(y)} &= \frac{d}{\cosh{(x)}} \\ y &= \arcsin{\left[\frac{d}{\cosh{(x)}}\right]} \end{align*}

Now you need to plot this.