How to Combine Resistors in a Triangular/Diamond Formation?

[Cursed]

Homework Statement

Find the Thevenin equivalent seen by R_L in the figure below:
[PLAIN]http://img708.imageshack.us/img708/7023/thevenineqi.png

Homework Equations

R_L = Load resistor

The double-headed arrows indicate that R_L is the load

The Attempt at a Solution

I know how to find the Thevenin equivalent in general, but I don't know how to combine the resistors in this problem because they're in a triangular / diamond formation. Can someone help me get started? I don't know which resistors are in parallel and which are in series.

Thanks!

Edit: The answers are Vt = 10V and Rt = 30 ohms

 

[zgozvrm]

This requires a wye-delta transformation.

When you have 3 resistors in a triangular formation with 3 connection points, you need to find a Y-shaped equivalent. That is, one with 3 resistors in a Y-formation such that the resistance from point-to-point remains the same as the original triangular formation.

See:
http://www.play-hookey.com/dc_theory/deriving_delta_y_conversions.html

 

[vij]

To find the Thevenin impedance you have to short circuit the voltage source. It seems the load resistor R_L is obstructing your imagination. Forget about R_L. The short circuit across the voltage source connects the junction of the 30 Ω and 20 Ω resistors to the junction of the 60 Ω and 20 Ω resistors. So you have the parallel combination of the 30 Ω and 60 Ω resistors in series with the parallel combination of the 20 Ω and 20 Ω resistors.
To find the Thevenin voltage also forget about RL with the 60 volt source in place. Find the voltage at the junction of the 30 Ω and 60 Ω resistors (with respect to the negative terminal of the battery. Similarly, find the voltage at the junction of the 20 Ω and 20 Ω resistors. The difference between these voltages is the Thevenin voltage.
Now, can’t you get the Thevenin equivalent of the given cicuit?