# Simplification of complex expression

## Homework Statement

For the expression:

$$E=\frac{E_{0}}{2}\left(\exp\left[\frac{j\pi V}{2V_{\pi}}\right]+j\exp\left[-\frac{j\pi V}{2V_{\pi}}\right]\right)$$

I want to show that if ##V=m(t)-\frac{V_{\pi}}{2}##, then ##|E|^2## can be written as:

$$|E|^2=\frac{E^2_{0}}{2}\left(1-\cos\left(\frac{\pi m(t)}{V_{\pi}}\right)\right). \tag{1}$$

Note: here ##j^2=-1##.

## The Attempt at a Solution

Substituting:

$$E(t)=\frac{E_{0}}{2}\left(\exp\left[\frac{j\pi}{2V_{\pi}}\left(m(t)-\frac{V_{\pi}}{2}\right)\right]+j\exp\left[-\frac{j\pi}{2V_{\pi}}\left(m(t)-\frac{V_{\pi}}{2}\right)\right]\right)$$

$$=\frac{E_{0}}{2}\left(\exp\left[j\left(\frac{\pi}{2V_{\pi}}m(t)-\frac{\pi}{4}\right)\right]+j\exp\left[-j\left(\frac{\pi}{2V_{\pi}}m(t)-\frac{\pi}{4}\right)\right]\right)$$

Multiplying by the complex conjugate:

##|E(2)|^{2}=\left(\frac{E_{0}}{2}\right)^{2}\left(\exp\left[j\left(\frac{\pi}{2V_{\pi}}m(t)-\frac{\pi}{4}\right)\right]+j\exp\left[-j\left(\frac{\pi}{2V_{\pi}}m(t)-\frac{\pi}{4}\right)\right]\right).\left(\exp\left[j\left(\frac{\pi}{2V_{\pi}}m(t)-\frac{\pi}{4}\right)\right]-j\exp\left[-j\left(\frac{\pi}{2V_{\pi}}m(t)-\frac{\pi}{4}\right)\right]\right)##

$$|E(2)|^{2}=\underline{\left(\frac{E_{0}}{2}\right)^{2}\left(\exp\left[j\left(\frac{\pi m(t)}{V_{\pi}}-\frac{\pi}{2}\right)\right]+\exp\left[-j\left(\frac{\pi m(t)}{V_{\pi}}-\frac{\pi}{2}\right)\right]\right)}.$$

Writing this explicitly in terms of trigonometric functions:

##=\left(\frac{E_{0}}{2}\right)^{2}\left[\left(\cos\left(\frac{\pi m(t)}{V_{\pi}}\right)+j\sin\left(\frac{\pi m(t)}{V_{\pi}}\right)\right)\left(\underbrace{\cos\left(-\frac{\pi}{2}\right)+j\sin\left(-\frac{\pi}{2}\right)}_{-j}\right)+\left(\cos\left(-\frac{\pi m(t)}{V_{\pi}}\right)+j\sin\left(-\frac{\pi m(t)}{V_{\pi}}\right)\right)\underbrace{\left(\cos\left(\frac{\pi}{2}\right)+j\sin\left(\frac{\pi}{2}\right)\right)}_{j}\right]##

$$=\left(\frac{E_{0}}{2}\right)^{2}\left[-j\cos\left(\frac{\pi m(t)}{V_{\pi}}\right)+\sin\left(\frac{\pi m(t)}{V_{\pi}}\right)+j\cos\left(\frac{\pi m(t)}{V_{\pi}}\right)+\sin\left(\frac{\pi m(t)}{V_{\pi}}\right)\right]$$

$$=\boxed{\frac{E_{0}^{2}}{2}\sin\left(\frac{\pi m(t)}{V_{\pi}}\right)}\stackrel{?}{=}\frac{E_{0}^{2}}{2}\left(1-\cos\left(\frac{\pi m(t)}{V_{\pi}}\right)\right)$$

If we had sin2, then we might have been able to use the half-angle formula. But I am not sure what to do here.

So, how can I get from ##\frac{E_{0}^{2}}{2}\sin\left(\frac{\pi m(t)}{V_{\pi}}\right)## to equation (1)? Did I make a mistake somewhere?

Any help is greatly appreciated.

mfb
Mentor
Multiplying by the complex conjugate:
There are sign errors here. What is the complex conjugate of ##e^{jx}##?

roam
Homework Helper
Gold Member
2020 Award
Suggestion to make it much simpler: Let ## m'=\frac{\pi m}{2V_{\pi}} ##. Also write ## je^{jx} ## as ## e^{j (\pi/2)} e^{jx} ##. A few minutes of work including correctly taking complex conjugates should get you the result.

roam and mfb
I see. I made a mistake taking the complex conjugate of the expression. So I used:

$$\overline{\exp\left[j\left(\frac{\pi m}{2V_{\pi}}-\frac{\pi}{4}\right)\right]+j\exp\left[-j\left(\frac{\pi m}{2V_{\pi}}-\frac{\pi}{4}\right)\right]}=\exp\left[-j\left(\frac{\pi m}{2V_{\pi}}-\frac{\pi}{4}\right)\right]-j\exp\left[j\left(\frac{\pi m}{2V_{\pi}}-\frac{\pi}{4}\right)\right]$$

and I got the correct result. Thank you so much for the suggestions.