Find the expression for Power in circuit

[VitaminK]

Homework Statement

Find the expression for Power in a Circuit with two light bulbs ( each with resistance R) in series. The terminal voltage is V.

Relevant Equations

Req= R1+ R2
P=V^2/R

I was wondering if someone could check my answer and see if it is correct.
The resistance in series is Req= R+ R= 2R

The voltage across 2R is the terminal voltage V. The expression I get is:

Can I also use this formula P=I^2*R?
When using it I get:
Current through the resistors is

The resistance in series is Req= R+ R= 2R
The Power is:

However i don't get the same answer :(

 

[Merlin3189]

Actually, you do get the same answer. ## \frac{V1^2\times 2}{R} = \frac {V^2}{2R}##

But why do the second calculation for just one lamp?
If you do the same as your first calculation for both lamps, 2R, you get a simpler answer.
But it is exactly the same as the first result!

You could also use P=IR and again get the same result.
It's hardly surprising: all you know is V and R, so you can't do better than P in terms of V and R.