# Is it wrong to assume the op-amp has the same voltage at its both terminals when deriving its non-inverting closed-loop gain?

I am referring to the approach taken in this video, at about 00:43. I know that the op-amp "tries" to balance the voltages at its both terminals but is it a completely correct approach to take V+ = V- when deriving the op-amp's closed-loop gain? As far as I know, in practice, V+ is never equal to V-. But from a theoretical point of view, would it always be correct to assume V+ = V- when calculating the op-amp's closed-loop gain?

• If the question specifies $A_{OL}$ then you deserve to lose points by assuming V+ = V-. – Spehro Pefhany Dec 3 '16 at 18:48