Question

Op amps for everyone!

Answer 1

First circuit:
http://www.calvin.edu/~svleest/circuitExamples/OpAmps/
for KCL I have
\(\frac{V_{in}-V_n}{R_1}+\frac{V_n-V_{out}}{R_2}=0\)
of course this is wrong and it should actually be
\(\frac{V_{in}-V_n}{R_1}+\frac{V_{out}-V_n}{R_2}=0\)
Via the solution
http://www.calvin.edu/~svleest/circuitExamples/OpAmps/soln1.htm
How do I know which sign is which?

Answer 2

\[i = \dfrac{V_a-V_b}{R}\]