# How to notate a variable that is being held constant

1. May 30, 2013

### jldibble

I remember learning at one point that you can put some sort of symbol on top or near a variable to indicate that it is being held constant. I thought that it was just a bar drawn above the variable but in some cases, it means "the average of."

Anyone know what I'm talking about?

2. May 30, 2013

### Mandelbroth

Do you mean that, when taking a partial derivative and want to specifically keep t constant, you can write, for example, $\left(\frac{\partial f}{\partial x}\right)_t$?

3. May 30, 2013

### jldibble

No, I mean something much simpler. It's a symbol or notation that you'd use as you're explaining how variables will change relative to each other.

Here's a very simple example using the equation d=vt (distance = speed*time):

Say I want to hold the speed at a constant value; I now know that by increasing the amount of time, I'll increase the value of the distance by a proportionate amount. My question involves a short-hand symbol for indicating that the speed is the variable that's being held constant, instead of wasting time writing out "___ is constant" everytime I'm discussing an equation.

4. May 30, 2013

### Vorde

In spreadsheet programs like excel this is done by putting '$' '$' around the variable, though I doubt that's what you're looking for.

Do you mean just saying:
$D= \frac{v}{t} \rightarrow D= \frac{v_1}{t}$? Because that sort of does it.

5. May 30, 2013

### jldibble

That's more like it, but not exactly what I had in mind.

It's such a dumb little thing but it's driving me nuts.

6. May 30, 2013

### Vorde

What class was it? That might help?

7. May 30, 2013

### a1call

8. May 30, 2013

### jldibble

It's not for a specific class. I tutor for math and physics and there are times when I want to denote which variable in the equation is being held constant for the sake of discussing how the other variables effect each other.

I remember a math (or science) teacher I had in high school making a mark near the variable that would be held constant.

9. May 30, 2013

### Mute

I'm not familiar with any standard notation that denotes what you want. You could make one up for the students you are tutoring, but let them know it's not a notation they're likely to see elsewhere.

You could say something like "let d = x(t) = vt", where the function notation x(t) explicitly denotes that t is the variable. If the students need a mark to remind them which symbols are constants, function notation may at first be confusing for them, but it's a notation they will likely see again in the future.

10. May 31, 2013

### jbriggs444

If you are talking about the distinction between "dependent variables" and "independent variables", I am not familiar with any specific notation, but there is a common convention of formulating equations so that the dependent variable goes on the left and the independent variable or variables go on the right.

Last edited: May 31, 2013