After I calculated that $$v_s=v_u\left(\frac{R_1}{R_1+R_2}\right)$$ I have to calculate the resistance seen by the voltage generator \$v_s\$.
My book, without any calculation, says it is: \$+\infty\$. Now, I am trying to figure out why.
I thought that the resistance seen by \$v_s\$ is $$\frac{v_s}{\frac{e_1-v_s}{R_s}}$$ (with the nodal analysis).
I tried to do algebraic calculations, but the results doesn't come out as \$+\infty\$. How should I calculate it?
simulate this circuit – Schematic created using CircuitLab
EDIT: If ideal op. amplifiers have infinite input resistance, so should be in the following circuit:
My book says in this last circuit it's \$R_1\$ (without any calc. too). Why?