### Author Topic: Q5 TUT 5201  (Read 4316 times)

#### Victor Ivrii

• Administrator
• Elder Member
• Posts: 2607
• Karma: 0
##### Q5 TUT 5201
« on: November 02, 2018, 03:33:36 PM »
Find the power-series expansion about the given point for the given function; find the largest disc in which the series is valid:
$$\frac{z+2}{z+3}\qquad\text{about}\; z_0 = -1.$$

#### Tianfangtong Zhang

• Full Member
• Posts: 16
• Karma: 15
##### Re: Q5 TUT 5201
« Reply #1 on: November 02, 2018, 03:41:27 PM »
\begin{align*}
\frac{z+2}{z+3} &= \frac{z+3-1}{z+3} \\ &= 1-\frac{1}{z+3}\\
&= 1- \frac{1}{z+1+2}\\ &= 1 - \frac{1}{2} \frac{1}{1+\frac{z+1}{2}}\\
&= 1 - \frac{1}{2} \frac{1}{1-\frac{-(z+1)}{2}}\\
&= 1 - \sum_{n=0}^{\infty}(\frac{-(z+1)}{2})^n \\
&= 1 - \sum_{n=0}^{\infty}\frac{(-1)^n}{2^{n+1}}(z-(-1))^n
\end{align*}

#### ZhenDi Pan

• Jr. Member
• Posts: 10
• Karma: 20
##### Re: Q5 TUT 5201
« Reply #2 on: November 02, 2018, 04:52:51 PM »
We can first rewrite $\frac{z+2}{z+3}$ as

\frac{z+2}{z+3} = \frac{z+3-1}{z+3}

\begin{align*}
\frac{z+3-1}{z+3} & = 1 - \frac{1}{z+3} \\
& = 1 - \frac{1}{z+1+2} \\
& = 1 - \frac{1}{2} \times \frac{1}{1+\frac{(z+1)}{2}} \\
& = 1 - \frac{1}{2} \times \frac{1}{1-\frac{-(z+1)}{2}}
\end{align*}
The power series expansion is

1 - \frac{1}{2} \times \frac{1}{1-\frac{-(z+1)}{2}} = 1 - \frac{1}{2} \times \sum_{n=0}^{\infty} (\frac{-(z+1)}{2})^n \\
= 1 - \sum_{n=0}^{\infty} \frac{(-1)^n}{2^{n+1}}(z-(-1))^n = 1 - \sum_{n=0}^{\infty} \frac{(-1)^n}{2^{n+1}}(z+1)^n

The largest disc in which the series is valid:

\mid -\frac{z+1}{2}\mid < 1 \\
\mid z+1\mid < 2

#### Heng Kan

• Full Member
• Posts: 15
• Karma: 18
##### Re: Q5 TUT 5201
« Reply #3 on: November 02, 2018, 07:46:50 PM »
See the attatched scanned picture. A different way to get the expansion.