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− | < | + | <center><font size= 4> |
− | + | '''[[Digital_signal_processing_practice_problems_list|Practice Question on "Digital Signal Processing"]]''' | |
− | + | </font size> | |
− | + | Topic: Computing an inverse z-transform | |
+ | </center> | ||
---- | ---- | ||
+ | ==Question== | ||
Compute the inverse z-transform of | Compute the inverse z-transform of | ||
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Therefore, <span class="texhtml">''x''[''n''] = 3<sup> − 1 + ''n''</sup>''u''[ − ''n'']</span> | Therefore, <span class="texhtml">''x''[''n''] = 3<sup> − 1 + ''n''</sup>''u''[ − ''n'']</span> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
+ | |||
=== Answer 2 === | === Answer 2 === | ||
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<span class="texhtml">''x''[''n''] = 3<sup>''n'' − 1</sup>''u''[ − ''n'']</span> | <span class="texhtml">''x''[''n''] = 3<sup>''n'' − 1</sup>''u''[ − ''n'']</span> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
+ | |||
=== Answer 3 === | === Answer 3 === | ||
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Using the z-transform formula, <span class="texhtml">''x''[''n''] = 3<sup>''n'' − 1</sup>''u''[ − ''n'']</span> | Using the z-transform formula, <span class="texhtml">''x''[''n''] = 3<sup>''n'' − 1</sup>''u''[ − ''n'']</span> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
=== Answer 4 === | === Answer 4 === | ||
− | + | Gena Xie | |
− | < | + | <math>X(z) = \frac{1}{3-Z} </math> |
+ | |||
+ | since |z|<3, | ||
+ | |||
+ | |z|/3 < 1 | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \frac{1} { 1-\frac{Z}{3}} </math> | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=0}^{+\infty} (\frac{z}{3})^n = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[n](\frac{z}{3})^n </math> | ||
+ | |||
+ | substitute n by -n, | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[-n](\frac{z}{3})^{-n} = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[-n](\frac{1}{3})^{-n} Z^{-n}</math> | ||
+ | |||
+ | based on the definition | ||
+ | |||
+ | <math>x[n] = \frac{1}{3} u[-n](\frac{1}{3})^{-n} = (\frac{1}{3})^{-n+1} u[-n] </math> | ||
<br> | <br> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
=== Answer 5 === | === Answer 5 === | ||
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<span class="texhtml">''x''[''n''] = 3<sup>''n'' − 1</sup>''u''[ − ''n'']</span> | <span class="texhtml">''x''[''n''] = 3<sup>''n'' − 1</sup>''u''[ − ''n'']</span> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
---- | ---- | ||
− | Answer 6 - Ryan Atwell | + | Answer 6 - Ryan Atwell |
<math>X(z) =\frac{1}{3-z}, \quad \text{ROC} \quad |z|<3 </math> | <math>X(z) =\frac{1}{3-z}, \quad \text{ROC} \quad |z|<3 </math> | ||
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<math>X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}} </math> | <math>X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}} </math> | ||
− | <math>X(z) = \frac{1}{3} \sum_{n=0}^{\infty} (\frac{z}{3})^{n} </math> | + | <math>X(z) = \frac{1}{3} \sum_{n=0}^{\infty} (\frac{z}{3})^{n} </math> |
− | <math>X(z) = \frac{1}{3} \sum_{n=-\infty}^{\infty} u[n](\frac{z}{3})^{n}</math> | + | <math>X(z) = \frac{1}{3} \sum_{n=-\infty}^{\infty} u[n](\frac{z}{3})^{n}</math> |
− | n=-k | + | n=-k |
+ | <br> <math>X(z) = \frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k](\frac{z}{3})^{-k}</math> | ||
− | <math>X(z) = \frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k] | + | <math>X(z) = \frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k]{3}^{k}{z}^{-k}</math> |
− | <math>X(z) = | + | <br> <math>X(z) = \sum_{k=-\infty}^{\infty} u[-k]{3}^{k-1}{z}^{-k}</math> |
+ | by formula | ||
− | < | + | <span class="texhtml">''X''[''n''] = 3<sup>''n'' − 1</sup>''u''[ − ''n'']</span> |
− | + | :<span style="color:blue"> Grader's comment: Correct Answer </span> | |
− | <math>X[n]={3}^{n-1}u[-n]</math> | + | ---- |
+ | |||
+ | Answer 7 | ||
+ | |||
+ | <math>X(z) = \frac{1}{3-z}</math> | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}}</math> | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \frac{1-\left( \frac{z}{3} \right) ^n}{1- \frac{z}{3}} </math> <math>, \quad \text{As n goes to} \quad \infty \quad \text{since} \quad |z|<3</math> | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=0}^{\infty} \left( \frac{z}{3} \right)^n</math> | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=-\infty}^{0} \left( \frac{1}{3} \right)^{-n} z^{-n}</math> | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=-\infty}^{\infty} \left(\frac{1}{3} \right)^{-n} u[-n] z^{-n}</math> <math>, \quad \text{By comparing with DTFT equation, we get} \quad</math> | ||
+ | |||
+ | <math>x[n] = \frac{1}{3} * \frac{1}{3}^{-n} u[-n]</math> | ||
+ | |||
+ | <math>x[n] = \left( \frac{1}{3} \right) ^{-n+1} u[-n]</math> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
---- | ---- | ||
− | |||
− | + | Answer 8 | |
− | <math>X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}}</math> | + | <math>X(z) = \frac{1}{3-z}</math> |
+ | |||
+ | <math>=\frac{1}{3}\frac{1}{1-\frac{z}{3}}</math> | ||
+ | |||
+ | <math>\frac{1}{3}\sum_{n=-\infty}^{\infty} u[n](\frac{z}{3})^n</math> | ||
+ | |||
+ | let n=-k | ||
+ | |||
+ | <math>X(z)=\frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k]3^k z^{-k}</math> | ||
+ | |||
+ | by comparison to inverse z-transform formula, | ||
+ | |||
+ | <span class="texhtml">''x''[''n''] = 3<sup> − 1 + ''k''</sup>''u''[ − ''k'']</span> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Right hand side is in 'k' while the left hand side in 'n' </span> | ||
+ | |||
+ | ---- | ||
+ | [[2013 Fall ECE 438 Boutin|Back to ECE438 Fall 2013 Prof. Boutin]] </math> | ||
− | |||
---- | ---- | ||
− | [[ | + | |
− | </math> | + | Answer 8 |
+ | |||
+ | <math>X(z) = \frac{1}{3-z}</math> | ||
+ | |||
+ | we have |z| < 3, so | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}} </math> | ||
+ | |||
+ | sum will look like this: | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=0}^{\infty} (\frac{z}{3})^{n} </math> | ||
+ | |||
+ | with unit step: | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=-\infty}^{\infty} u[n](\frac{z}{3})^{n}</math> | ||
+ | |||
+ | substituting n with -k we get: | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k](\frac{z}{3})^{-k}</math> | ||
+ | |||
+ | <br> finally we get: | ||
+ | |||
+ | <math>X(z) = \sum_{k=-\infty}^{\infty} u[-k]{3}^{k-1}{z}^{-k}</math> | ||
+ | |||
+ | using the formula we get: | ||
+ | |||
+ | <math>x[n] = (\frac{1}{3})^{-n+1} u[-n] </math> | ||
+ | |||
+ | <br> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
+ | |||
+ | === Answer 9 === | ||
+ | |||
+ | <math> X(z) = \frac{(\frac{1}{3})}{1-(\frac{z}{3})} </math> | ||
+ | |||
+ | <br> <math> X(z) = (\frac{1}{3})*\sum_{n=0}^{\infty} \frac{1}{1-(\frac{z}{3})} </math> | ||
+ | |||
+ | <br> <math> X(z) = (\frac{1}{3})*\sum_{n=-\infty}^{\infty} u[n] (\frac{z}{3})^{n} </math> | ||
+ | |||
+ | <br> let -k = n, | ||
+ | |||
+ | <br> <math> X(z) = (\frac{1}{3})*\sum_{n=-\infty}^{\infty} u[-k] (\frac{z}{3})^{-k} </math> | ||
+ | |||
+ | <br> <math> X(Z) = (\frac{1}{3})*\sum_{n=-\infty}^{\infty} u[n] (\frac{1}{3})^{-k}* Z^{-k} </math> | ||
+ | |||
+ | <br> so by comparison <math> , x[n] = (\frac{1}{3})^{-n+1} u[-n] </math> | ||
+ | |||
+ | <br> | ||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
+ | |||
+ | === Answer 10 === | ||
+ | |||
+ | <math>X(z) =\frac{1}{3-z} </math>. | ||
+ | |||
+ | <math>X(z) =(\frac{1}{3}) \frac{1}{1-\frac{z}{3}} </math> | ||
+ | |||
+ | Since z/3 < |1|, base on geometric series: | ||
+ | |||
+ | <math> \frac{1}{1-\frac{z}{3}} = \sum_{n=0}^{\infty} (\frac{z}{3})^{n} </math> | ||
+ | |||
+ | <math>X(z) =(\frac{1}{3}) \sum_{n=0}^{\infty} (\frac{z}{3})^{n} = \sum_{n=0}^{\infty} z^{n} (\frac{1}{3})^{n+1}</math> | ||
+ | |||
+ | <math>X(z) =\sum_{n=-\infty}^{\infty} u[n] z^{n} (\frac{1}{3})^{n+1}</math> | ||
+ | |||
+ | Let n = -m, <math>X(z) =\sum_{-m=-\infty}^{\infty} u[-m] z^{-m} (\frac{1}{3})^{-m+1}</math> | ||
+ | |||
+ | <math>X(z) =\sum_{m=-\infty}^{\infty} {z}^{-m} u[-m]{3}^{m-1}</math> | ||
+ | |||
+ | base on observation: <span class="texhtml">''x''[''n''] = ''u''[ − ''n'']3<sup>''n'' − 1</sup></span> | ||
+ | |||
+ | <br> | ||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
+ | |||
+ | === Answer 11<br> === | ||
+ | |||
+ | |||
+ | <math>X(z) =\frac{1}{3-z} </math> <math>X(z) =(\frac{1}{3}) \frac{1}{1-\frac{z}{3}} </math> | ||
+ | |||
+ | for <math>\quad \text{ROC} \quad |z|<3 </math> | ||
+ | |||
+ | we can use geometric series | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{n=0}^{+\infty} (\frac{z}{3})^n </math><br> <math>X(z) = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[n](\frac{z}{3})^n </math> | ||
+ | |||
+ | <math>X(z) = \frac{1}{3} \sum_{k=-\infty}^{+\infty} u[-k](\frac{1}{3})^{-k} Z^{-k}</math> | ||
+ | |||
+ | so, by comparing to z-transform formula,we have | ||
+ | |||
+ | <br> <math> x[n] = (\frac{1}{3})^{-n+1} u[-n] </math> <br> | ||
+ | |||
+ | :<span style="color:blue"> Grader's comment: Correct Answer </span> | ||
+ | |||
+ | <br> | ||
+ | |||
[[Category:ECE301]] [[Category:ECE438]] [[Category:ECE438Fall2013Boutin]] [[Category:Problem_solving]] [[Category:Z-transform]] [[Category:Inverse_z-transform]] | [[Category:ECE301]] [[Category:ECE438]] [[Category:ECE438Fall2013Boutin]] [[Category:Problem_solving]] [[Category:Z-transform]] [[Category:Inverse_z-transform]] |
Latest revision as of 11:54, 26 November 2013
Practice Question on "Digital Signal Processing"
Topic: Computing an inverse z-transform
Contents
Question
Compute the inverse z-transform of
$ X(z) =\frac{1}{3-z}, \quad \text{ROC} \quad |z|<3 $.
(Write enough intermediate steps to fully justify your answer.)
You will receive feedback from your instructor and TA directly on this page. Other students are welcome to comment/discuss/point out mistakes/ask questions too!
Answer 1
$ X[z] = \sum_{n=0}^{+\infty} 3^{-1-n} z^{n} $
$ = \sum_{n=-\infty}^{+\infty} u[n] 3^{-1-n} z^{n} $
NOTE: Let n=-k
$ = \sum_{n=-\infty}^{+\infty} u[-k] 3^{-1+k} z^{-k} $ (compare with $ \sum_{n=-\infty}^{+\infty} x[n] z^{-k} $)
$ = \sum_{n=-\infty}^{+\infty} u[-k] 3^{-1+k} z^{-k} $
Therefore, x[n] = 3 − 1 + nu[ − n]
- Grader's comment: Correct Answer
Answer 2
$ X(z) = \frac{1}{3-z} = \frac{1}{3} \frac{1}{1-\frac{z}{3}} $
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[n] (\frac{z}{3})^n $
Let n = -k
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[-k]3^{k}z^{-k} $
By comparison with the x-transform formula,
x[n] = 3n − 1u[ − n]
- Grader's comment: Correct Answer
Answer 3
By Yeong Ho Lee
$ X[z] = \sum_{n=0}^{\infty}z^{n}3^{-n-1} $
$ = \sum_{n=-\infty}^{+\infty} 3^{-n-1}z^{n}u[n] $
Now, let n = -k
$ = \sum_{n=-\infty}^{+\infty} 3^{k-1}z^{-k}u[-k] $
Using the z-transform formula, x[n] = 3n − 1u[ − n]
- Grader's comment: Correct Answer
Answer 4
Gena Xie
$ X(z) = \frac{1}{3-Z} $
since |z|<3,
|z|/3 < 1
$ X(z) = \frac{1}{3} \frac{1} { 1-\frac{Z}{3}} $
$ X(z) = \frac{1}{3} \sum_{n=0}^{+\infty} (\frac{z}{3})^n = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[n](\frac{z}{3})^n $
substitute n by -n,
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[-n](\frac{z}{3})^{-n} = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[-n](\frac{1}{3})^{-n} Z^{-n} $
based on the definition
$ x[n] = \frac{1}{3} u[-n](\frac{1}{3})^{-n} = (\frac{1}{3})^{-n+1} u[-n] $
- Grader's comment: Correct Answer
Answer 5
By Yixiang Liu
$ X(z) = \frac{1}{3-Z} $
$ X(z) = \frac{\frac{1}{3}} { 1-\frac{Z}{3}} $
$ X(z) = \frac{1}{3} * \frac{1} { 1-\frac{Z}{3}} $
by geometric series
$ X(z) = \frac{1}{3} \sum_{n=0}^{+\infty} (\frac{z}{3})^n $
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[n](\frac{z}{3})^n $
$ X(z) = \frac{1}{3} \sum_{k=-\infty}^{+\infty} u[-k](\frac{z}{3})^{-k} $
$ X(z) = \frac{1}{3} \sum_{k=-\infty}^{+\infty} u[-k](\frac{1}{3})^{-k} Z^{-k} $
By comparison with the x-transform formula
$ x[n] = \frac{1}{3} u[-n](\frac{1}{3})^{-n} $
$ x[n] = (\frac{1}{3})^{-n+1} u[-n] $
x[n] = 3n − 1u[ − n]
- Grader's comment: Correct Answer
Answer 6 - Ryan Atwell
$ X(z) =\frac{1}{3-z}, \quad \text{ROC} \quad |z|<3 $
$ X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}} $
$ X(z) = \frac{1}{3} \sum_{n=0}^{\infty} (\frac{z}{3})^{n} $
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{\infty} u[n](\frac{z}{3})^{n} $
n=-k
$ X(z) = \frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k](\frac{z}{3})^{-k} $
$ X(z) = \frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k]{3}^{k}{z}^{-k} $
$ X(z) = \sum_{k=-\infty}^{\infty} u[-k]{3}^{k-1}{z}^{-k} $
by formula
X[n] = 3n − 1u[ − n]
- Grader's comment: Correct Answer
Answer 7
$ X(z) = \frac{1}{3-z} $
$ X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}} $
$ X(z) = \frac{1}{3} \frac{1-\left( \frac{z}{3} \right) ^n}{1- \frac{z}{3}} $ $ , \quad \text{As n goes to} \quad \infty \quad \text{since} \quad |z|<3 $
$ X(z) = \frac{1}{3} \sum_{n=0}^{\infty} \left( \frac{z}{3} \right)^n $
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{0} \left( \frac{1}{3} \right)^{-n} z^{-n} $
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{\infty} \left(\frac{1}{3} \right)^{-n} u[-n] z^{-n} $ $ , \quad \text{By comparing with DTFT equation, we get} \quad $
$ x[n] = \frac{1}{3} * \frac{1}{3}^{-n} u[-n] $
$ x[n] = \left( \frac{1}{3} \right) ^{-n+1} u[-n] $
- Grader's comment: Correct Answer
Answer 8
$ X(z) = \frac{1}{3-z} $
$ =\frac{1}{3}\frac{1}{1-\frac{z}{3}} $
$ \frac{1}{3}\sum_{n=-\infty}^{\infty} u[n](\frac{z}{3})^n $
let n=-k
$ X(z)=\frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k]3^k z^{-k} $
by comparison to inverse z-transform formula,
x[n] = 3 − 1 + ku[ − k]
- Grader's comment: Right hand side is in 'k' while the left hand side in 'n'
Back to ECE438 Fall 2013 Prof. Boutin </math>
Answer 8
$ X(z) = \frac{1}{3-z} $
we have |z| < 3, so
$ X(z) = \frac{1}{3} \frac{1}{1-\frac{z}{3}} $
sum will look like this:
$ X(z) = \frac{1}{3} \sum_{n=0}^{\infty} (\frac{z}{3})^{n} $
with unit step:
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{\infty} u[n](\frac{z}{3})^{n} $
substituting n with -k we get:
$ X(z) = \frac{1}{3} \sum_{k=-\infty}^{\infty} u[-k](\frac{z}{3})^{-k} $
finally we get:
$ X(z) = \sum_{k=-\infty}^{\infty} u[-k]{3}^{k-1}{z}^{-k} $
using the formula we get:
$ x[n] = (\frac{1}{3})^{-n+1} u[-n] $
- Grader's comment: Correct Answer
Answer 9
$ X(z) = \frac{(\frac{1}{3})}{1-(\frac{z}{3})} $
$ X(z) = (\frac{1}{3})*\sum_{n=0}^{\infty} \frac{1}{1-(\frac{z}{3})} $
$ X(z) = (\frac{1}{3})*\sum_{n=-\infty}^{\infty} u[n] (\frac{z}{3})^{n} $
let -k = n,
$ X(z) = (\frac{1}{3})*\sum_{n=-\infty}^{\infty} u[-k] (\frac{z}{3})^{-k} $
$ X(Z) = (\frac{1}{3})*\sum_{n=-\infty}^{\infty} u[n] (\frac{1}{3})^{-k}* Z^{-k} $
so by comparison $ , x[n] = (\frac{1}{3})^{-n+1} u[-n] $
- Grader's comment: Correct Answer
Answer 10
$ X(z) =\frac{1}{3-z} $.
$ X(z) =(\frac{1}{3}) \frac{1}{1-\frac{z}{3}} $
Since z/3 < |1|, base on geometric series:
$ \frac{1}{1-\frac{z}{3}} = \sum_{n=0}^{\infty} (\frac{z}{3})^{n} $
$ X(z) =(\frac{1}{3}) \sum_{n=0}^{\infty} (\frac{z}{3})^{n} = \sum_{n=0}^{\infty} z^{n} (\frac{1}{3})^{n+1} $
$ X(z) =\sum_{n=-\infty}^{\infty} u[n] z^{n} (\frac{1}{3})^{n+1} $
Let n = -m, $ X(z) =\sum_{-m=-\infty}^{\infty} u[-m] z^{-m} (\frac{1}{3})^{-m+1} $
$ X(z) =\sum_{m=-\infty}^{\infty} {z}^{-m} u[-m]{3}^{m-1} $
base on observation: x[n] = u[ − n]3n − 1
- Grader's comment: Correct Answer
Answer 11
$ X(z) =\frac{1}{3-z} $ $ X(z) =(\frac{1}{3}) \frac{1}{1-\frac{z}{3}} $
for $ \quad \text{ROC} \quad |z|<3 $
we can use geometric series
$ X(z) = \frac{1}{3} \sum_{n=0}^{+\infty} (\frac{z}{3})^n $
$ X(z) = \frac{1}{3} \sum_{n=-\infty}^{+\infty} u[n](\frac{z}{3})^n $
$ X(z) = \frac{1}{3} \sum_{k=-\infty}^{+\infty} u[-k](\frac{1}{3})^{-k} Z^{-k} $
so, by comparing to z-transform formula,we have
$ x[n] = (\frac{1}{3})^{-n+1} u[-n] $
- Grader's comment: Correct Answer