Amplifiers homework problem

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1. The problem statement, all variables and given/known data



2. Relevant equations

[tex]dI = \frac{\partial I}{\partial V_{GS}} dV_{GS} + \frac{\partial I}{\partial V_{DS}}dV_{DS}[/tex]


3. The attempt at a solution

I was just wondering why [tex]\small V_{G}[/tex] is at 0. I have solved the entire question (there are more parts :P), but I could only do it once I was told that [tex]\small V_{GS} = - V_S + V_G = - V_S[/tex]. I am having a hard time getting my head around that.
Shouldn't [tex]\small V_{G}[/tex] simply be [tex]\small V_{i}[/tex]?