In[9]:= soln = Simplify[Solve[{ip == k*((vdd - vin - vt)*(vdd - vo[t]) - (vdd - vo[t])^2/2), vin == m*t}, {ip, vin}]] Out[9]= {{ip -> 1/2 k (vdd - vo[t]) (-2 m t + vdd - 2 vt + vo[t]), vin -> m t}}
I don't get the error message from Solve, so I suspect you had assigned a value to vin.
