Help deriving an equation.

I believe that the following two equations are equivalent, but I am unable to algebraically derive one from the other. For computing a running average, I believe the following formula is correct where n = number of values in oldMean:

.

Similarly, I found this equation which I think does the same thing:

.

Are these equations in fact equivalent? They both seem to work when calculating a running average. If they are equivalent, how do I derive one from the other?

Thank you. This seems like an easy problem, but I've managed to confuse myself.

Re: Help deriving an equation.

Let's denote oldMean by m, newMean by m' and newVal by v. Then