Circuits with series and parallel wiring

[r_swayze]

Four identical 5.00 Ω resistors are joined and form the four sides of a square. (a) What is the resistance between points A and B? (b) What is the resistance between A and C?

The answer to (a) is 3.75 and (b) is 5, but I have no idea how to get those answers. Can somebody help?

 

[rl.bhat]

Do you know the equivalent resistance of the series and parallel combination of resistances? It not, search the web or the textbook.