I understand the voltage input at Rb gives the first two additions of the equations. But why is this subtracted from R3 and R4?
Electronic – Why does this dual summing op amp have this equation
operational-amplifier
Related Solutions
First, the DUT is in the positive feedback because this way you have an inverting and a non-inverting gain in the loop, so that multiplying them you obtain an overall negative loop gain.
Second, the gain is 1000 because Vin of DUT is Vos*50/(50K+50), so if you consider that V+ of the DUT should be 0, there is only the offset applied, so the feedback forces the output to be 1k times the offset voltage.
I think that you can look at the output this way: suppose that the situation is the one described, and you have a 500uV DUT offset and so 500mV output voltage.
Now, if you try to perturb the Servo Input, the feedback forces the output of the DUT to be almost the same of Servo Input, restoring the same Output voltage.
Note: Voff is the conventional name for input offset voltage, while Vos is the output voltage with Voff applied at the input pins.
Yes - I totally agree with Olin its a messed up circuit - just goes to show you shouldn't believe everything you read. I think the actual circuit should have been - 
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Best Answer
The equation is correct, although the unsimplified form would be clearer. The first stage sums in the inverting input, which means that those terms should be negative when summed in the second stage. And then it inverts in the second stage.
-(-(a + b) + c + d) = a + b - c - d