Need help calculating current and understanding Vos in an op amp

Question

I'm trying to solve a circuit that uses an op amp with a +-4mV offset but I've run into two problems trying to find output offset voltage.

1. When looking at the solution to the problem, I noticed that the solution had i1 current equal to (VA-Vos)/1Mohm. I don't understand why they made a separate node labeled VA because I had it originally as Vo-Vos/1Mohm. Is it because Vo value changes after going through the resistor so you label the new value VA? Also wanted to know how they label both currents through the resistors i1 because the first one on the left should be I=Vos/1 and the second should be I=VA-Vos/1

2. why is a voltage source included on the positive terminal when in the original the positive terminal was grounded.

First picture is original and second is the redrawn one .

Answer 1

1. Va is a separate node, which has a considerably different voltage than Vo. The input bias current of the op-amp is assumed to be zero, so the current i1 is common to the two resistors, and can be calculated as Vos/1M\$\Omega\$ or 4nA.

2. The Vos is the 4mV offset voltage represented as a voltage source on the non-inverting input of an otherwise ideal op-amp. It's internal to the op-amp or you can draw it as external to an ideal op-amp.