How can i calculate the voltage division in a circuit like this?

[CaspianTiger]

http://i5.photobucket.com/albums/y160/bornstubborn101/circuit3.jpg

This is the circuit. Basically i need to calculate the two resistors that would make up the potentiometer at VR1 to give a voltage of 3V.
I was going to use

VR1 = R2/(R1+R2)*Vs

The make:

R1= VR1/Vs*(R1+R2)

However apaprently this is wrong due to the +15V and the -15V inputs and that i need to use this equation.

R1=(0.5- VR1/VS)*(R1+R2)

I don;t know where this equation comes from so i can;t really use it in my report. Does anyone know where this equation comes from and how i can explain it.

Thanks

 

[MATLABdude]

I'm not quite sure I understand what you're saying when you need to calculate the two resistors that make up a potentiometer. A potentiometer is a resistor with three terminals--across the two end terminals, you have constant resistance while the wiper adjusts the position from one end to the other:
http://en.wikipedia.org/wiki/Potentiometer

In this case, R1 + R2 add up to the potentiometer resistance. Don't worry about the fact that there's +/- 15V at the top and bottom end of the potentiometer. There's a total of 30V across the resistor. A level of +3V is 18V from the bottom (the -15V). That should give you the hint you need to complete this.

 

[CaspianTiger]

I'm not quite sure I understand what you're saying when you need to calculate the two resistors that make up a potentiometer. A potentiometer is a resistor with three terminals--across the two end terminals, you have constant resistance while the wiper adjusts the position from one end to the other:
http://en.wikipedia.org/wiki/Potentiometer

In this case, R1 + R2 add up to the potentiometer resistance. Don't worry about the fact that there's +/- 15V at the top and bottom end of the potentiometer. There's a total of 30V across the resistor. A level of +3V is 18V from the bottom (the -15V). That should give you the hint you need to complete this.

Ok i understand the values of the resistors R1 and R2 are 4 and 6 ohms. However i am trying to use the final equation in my first post in a report i am writing. However i do not know where this equation comes from and i need to explain this potentiometer and the values for the R1 and R2 which make it.

 

[MATLABdude]

Ok i understand the values of the resistors R1 and R2 are 4 and 6 ohms. However i am trying to use the final equation in my first post in a report i am writing. However i do not know where this equation comes from and i need to explain this potentiometer and the values for the R1 and R2 which make it.

See the second paragraph of my response.