# Math Help - complex roots

1. ## complex roots

a) Show that $e^{(r_{1}+r_{2})t}=e^{r_{1}t}e^{r_{2}t}$ for any complex number $r_{1}$ and $r_{2}$.

b) Show that $\frac{d}{dt}e^{rt}=re^{rt}$.

For (a) I am stuck because $e^{(r_{1}+r_{2})t}=e^{(\lambda+i\mu+\lambda-i\mu)t}=e^{2\lambda\\t}$. I am not sure where to go from here. Should I just consider to arbitrary positive complex number rather than conjugate roots? I am considering roots because that is all that is talked about in the section.

For (b) do I just differentiate $e^{\lambda\\t}(cos(\mu\\t)+isin(\mu\\t)$ since $e^{(\lambda+i\mu)t}=e^{\lambda\\t}(cos(\mu\\t)+isi n(\mu\\t)$ or can I just look at the power series expansion of $e^{(\lambda+i\mu)t}$ and differentiate that?

2. Originally Posted by Danneedshelp
a) Show that $e^{(r_{1}+r_{2})t}=e^{r_{1}t}e^{r_{2}t}$ for any complex number $r_{1}$ and $r_{2}$.

b) Show that $\frac{d}{dt}e^{rt}=re^{rt}$.

For (a) I am stuck because $e^{(r_{1}+r_{2})t}=e^{(\lambda+i\mu+\lambda-i\mu)t}=e^{2\lambda\\t}$. I am not sure where to go from here. Should I just consider to arbitrary positive complex number rather than conjugate roots? I am considering roots because that is all that is talked about in the section.

For (b) do I just differentiate $e^{\lambda\\t}(cos(\mu\\t)+isin(\mu\\t)$ since $e^{(\lambda+i\mu)t}=e^{\lambda\\t}(cos(\mu\\t)+isi n(\mu\\t)$ or can I just look at the power series expansion of $e^{(\lambda+i\mu)t}$ and differentiate that?
Part (a)
Where does it say $r_1$ and $r_2$ are conjugates?

$r_1=a\pm b\mathbf{i}$
$r_2=c\pm d \mathbf{i}$

$e^{((a+b\mathbf{i})+(c+d\mathbf{i}))t}=e^{(a+b\mat hbf{i})t+(c+d\mathbf{i})t}=e^{(a+b\mathbf{i})t}e^{ (c+d\mathbf{i})t}=e^{r_1t}e^{r_2t}$