Find the phasor representation of an equation

[VinnyCee]

Homework Statement

Find the phasors of the following time functions:

(a) v(t)\,=\,3\,cos\left(\omega\,t\,-\,\frac{\pi}{3}\right)

(b) v(t)\,=\,12\,sin\left(\omega\,t\,+\,\frac{\pi}{4}\right)

(c) i(x,\,t)\,=\,2\,e^{-3\,x}\,sin\left(\omega\,t\,+\,\frac{\pi}{6}\right)

(d) i(t)\,=\,-2\,cos\left(\omega\,t\,+\,\frac{3\pi}{4}\right)

(e) i(t)\,=\,4\,sin\left(\omega\,t\,+\,\frac{\pi}{3}\right)\,+\,3\,cos\left(\omega\,t\,-\,\frac{\pi}{6}\right)

Homework Equations

A short list of conversions from a larger table in the book. These are conversions from time domain sinusoidal functions on the left to cosine-reference phasor functions on the right.

A\,cos\left(\omega\,t\,+\,\phi_0\right)\,\,\iff\,\,A\,e^{j\,\phi_0}

A\,sin\left(\omega\,t\,+\,\phi_0\right)\,\,\iff\,\,A\,e^{j\left(\phi_0\,-\,\frac{\pi}{2}\right)}

The Attempt at a Solution

(a) 3\,e^{-\frac{\pi}{3}\,j}

(b) 12\,e^{j\,\left(\frac{\pi}{4}\,-\,\frac{\pi}{2}\right)}\,=\,12\,e^{-\frac{\pi}{4}\,j}

(c) 2\,e^{-3\,x}\,e^{j\,\left(\frac{\pi}{6}\,-\,\frac{\pi}{2}\right)}\,=\,2\,e^{-3\,x}\,e^{-\frac{\pi}{3}\,j}\,=\,2\,e^{-3\,x\,-\,\frac{\pi}{3}\,j}

(d) -2\,e^{\frac{3\pi}{4}\,j}

(e) 4\,e^{j\,\left(\frac{\pi}{3}\,-\,\frac{\pi}{2}\right)}\,+\,3\,e^{j\,\left(-\frac{\pi}{6}\right)}\,=\,7\,e^{j\,\left(-\frac{\pi}{6}\right)}I have a question especially with (d), the answer is given as...

-2\,e^{j\,\left(\frac{3\pi}{4}\right)}\,=\,-2\,e^{j\,\left(\frac{\pi}{4}\right)}\,=\,2\,e^{-j\,\left(\frac{\pi}{4}\right)}

I don't understand how they did the last two conversions in the given answer! Can someone please explain, and say whether the others are correct as well?

Thanks

 

[Marco_84]

what you have to know is just euler formula: exp(ix)=cos(x)+isin(x).