<p>VCG's comment about isolating $k$ is the correct approach.</p>

<p>Given $U(\cdot) = k$ and
$$z = \frac{k^\frac{1}{\delta}}{x^\frac{\alpha}{\delta}y^\frac{\beta}{\delta}}$$
raise each side to the $\delta$ power:
$$z^\delta = \frac{k}{x^\alpha y^\beta}$$
and isolate $k$
$$k = x^\alpha y^\beta z^\delta$$</p>

<p>Since $k$ is now a function of each of the goods in your equation, it makes sense as a function form for utility.</p>

<p>$$U(x, y, z) = x^\alpha y^\beta z^\delta$$
So we arrive at the Cobb-Douglass utility form.</p>

CC BY-SA 3.0
2016-09-21T05:41:19.380Z
"I personally give answers that try not to reveal the whole solution, and merely demonstrate the setup (though perhaps I still give away too much :P). I like our current policy on hw." – [Kitsune Cavalry](http://meta.economics.stackexchange.com/a/1624/1601)
Giskard
CC BY-SA 3.0
2016-09-21T15:33:13.433Z
Yeah, either this question would have to be left with VCG's hint or someone would have to solve it. Sometimes I opt to solve questions. If the asker was really stuck, (just due to sheer unfamiliarity, as I've never seen a question made in this particular way before) I didn't feel like the hint would help much.
Kitsune Cavalry
CC BY-SA 3.0
2016-09-21T15:34:09.157Z
(This is the politically correct way of saying I'm a greedy boy who likes points right??? :P)
Kitsune Cavalry
CC BY-SA 3.0
2016-09-21T15:37:03.783Z
Guess I shoulda answered it then lol
VCG
CC BY-SA 3.0
2016-09-21T15:38:04.057Z
I personally wouldn't have minded. But whether you agree with my judgment to answer the question is for yo to decide.
Kitsune Cavalry
CC BY-SA 3.0
2016-09-21T02:48:05.863Z
2016-09-21T02:48:05.863Z
Kitsune Cavalry
<p>Given an indifference curve, how do you go about finding a utility function? </p>

<p>For example, given $z= \frac{k^\frac{1}{\delta}}{x^\frac{\alpha}{\delta}y^\frac{\beta}{\delta}}$ (defined by $U(\cdot) = k$), find a utility function. To do this, would I have to assign an arbitrary number for the utility and rewrite the function? I'm confused about what to do and can't find anything in my textbook about it. Thanks </p>

CC BY-SA 3.0
2016-09-21T01:10:51.330Z
I suspect the key points are that (1) points on the indifference curve must all have the same utility and (2) the utility function is monotonic in the right direction
Henry
CC BY-SA 3.0
2016-09-21T01:50:55.977Z
Is there a reason why you simply cannot rearrange to get k=U* as a function of x,y, and z?
VCG
<microeconomics><utility>
Find utility function given indifference curve?
Steven T
# Find utility function given indifference curve?

(Asked by: username_0 on 2016-09-21)

Given an indifference curve, how do you go about finding a utility function?

For example, given $z= \frac{k^\frac{1}{\delta}}{x^\frac{\alpha}{\delta}y^\frac{\beta}{\delta}}$ (defined by $U(\cdot) = k$), find a utility function. To do this, would I have to assign an arbitrary number for the utility and rewrite the function? I'm confused about what to do and can't find anything in my textbook about it. Thanks

**Tags:** microeconomics, utility

> **Comments:**
> **username_1**: I suspect the key points are that (1) points on the indifference curve must all have the same utility and (2) the utility function is monotonic in the right direction  
> **username_2**: Is there a reason why you simply cannot rearrange to get k=U* as a function of x,y, and z?  

---

## 1 answers

### Answer by username_3 on 2016-09-21. Score: 3 | Accepted answer

VCG's comment about isolating $k$ is the correct approach.

Given $U(\cdot) = k$ and
$$z = \frac{k^\frac{1}{\delta}}{x^\frac{\alpha}{\delta}y^\frac{\beta}{\delta}}$$
raise each side to the $\delta$ power:
$$z^\delta = \frac{k}{x^\alpha y^\beta}$$
and isolate $k$
$$k = x^\alpha y^\beta z^\delta$$

Since $k$ is now a function of each of the goods in your equation, it makes sense as a function form for utility.

$$U(x, y, z) = x^\alpha y^\beta z^\delta$$
So we arrive at the Cobb-Douglass utility form.

> **Comments:**
> **username_4**: "I personally give answers that try not to reveal the whole solution, and merely demonstrate the setup (though perhaps I still give away too much :P). I like our current policy on hw." – [username_3 username_3](http://meta.economics.stackexchange.com/a/1624/1601)  
> **username_3**: Yeah, either this question would have to be left with VCG's hint or someone would have to solve it. Sometimes I opt to solve questions. If the asker was really stuck, (just due to sheer unfamiliarity, as I've never seen a question made in this particular way before) I didn't feel like the hint would help much.  
> **username_3**: (This is the politically correct way of saying I'm a greedy boy who likes points right??? :P)  
> **username_2**: Guess I shoulda answered it then lol  
> **username_3**: I personally wouldn't have minded. But whether you agree with my judgment to answer the question is for yo to decide.  

---

