[Solved] Trying to solve polynomial system

Questions
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1

Hello, I am new to wolfram and the wolfram language. I am trying to solve a system of polynomial equations describing a very simple electrical circuit and I do not understand why this is not working. Solve is giving me no solutions. I am sure I must be missing something basic…
Feedback is welcome,
thanks!

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2

You must add a list of variables in Solve[] function, something like this:

eq1 = (IDSAT - ((1/2*mun0*Coxn*W/L)*((VGS - Vth)^2)*(1 + 
         lambdan*VDS)) == 0);
eq2 = (IRSS - (VX/RSS) == 0);
eq3 = (IRD - (VDD/RD) + (VOUT/RD) == 0);
eq4 = (VGS - VG + VX == 0);
eq5 = (VDS - VOUT + VX == 0);
eq6 = (VG - VIN == 0);
eq7 = (IDSAT - IRD == 0);
eq8 = (IRD - IRSS == 0);

eqns = {eq1, eq2, eq3, eq4, eq5, eq6, eq7, eq8};
Solve[eqns, {VOUT, IRD, IRSS, VX, VGS, VDS, VG, IDSAT}]

or:

eq1 = (ID == ((0.5mun0CoxnW/L)((VGS - Vth)^2)(1 + lambdanVOUT)));
eq2 = (IRD == ID);
eq3 = (IRD == (VDD - VOUT)/RD);

eqnsp2 = {eq1, eq2, eq3};
Solve[eqnsp2, {VOUT, IRD, ID}]

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3

You cannot solve a system of equation with respect to just one variable:

Solve[{x + y == 1, y - 1 == 2}, x]

because you cannot make the two equations true by just replacing the value of x. In your case you must give the full list of unknowns, or else let the system figure out what the unknowns are:

Solve[eqns]
4

Thanks for the reply.
This makes sense and it solved my issue.