Hi all, This is not HW, but I think this is the best section for my question. I was reading the pdf below, where I cannot full understand what are the steps followed in Q4 a and b. For Q4a I can see that KCL was used, but what are the steps followed from vi/R1 + (R2/R1 vi) 1/R3 till the end of the derivation? Also, in Q4a, for which resistor is he referring to when he wrote Ri and what are the steps he followed for this same question please? http://ece.uprm.edu/~mtoledo/5207/S2013/hw1/solhw1.pdf Thanks in advance.