MAT334--2020F > Quiz 5

Quiz-5-6101 A

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Jiaqi Bi:
$\textbf{Problem}$:
Give the order of each of the zeros of the given function:
$$e^{2z}-3e^z-4$$

$\textbf{Solution}$:
$f(z)=e^{2z}-3e^z-4=0$
Let $w=e^z$, we get $w^2-3w-4=(w-4)(w+1)$
Then resubstitute $e^z=w$
We get $(e^z-4)(e^z+1)=0 \Rightarrow e^z=4\ \text{and}\ e^z=-1$
Solve for $z$, we get $z=log(4)=ln(4)+i(2k\pi)\ \text{and}\ z=log(-1) =ln(-1)+i(2k\pi) =\pi i+i(2k\pi) =i(\pi+2k\pi)$
To find orders: $f'(z)=2e^{2z}-3e^z$
Substitute $e^z=4\ \text{and}\ -1$ into $f'(z)$
$2\times 16-3\times 4 = 20 \neq 0, 2+3=5\neq 0$
Since first derivative does not equal to $0$ for both $z$, we conclude order $= 1$
Hence, the order is $1$ for $z=ln(4)+i(2k\pi)\ \text{and}\ z=i(\pi+2k\pi)$