Circuit analysis on op-amp with no negative feedback
The source you are using is wrong.  You know because the units are wrong -- a voltage divided by a resistance is a current, not a voltage.  The correct expression is yielded using voltage division: \$\frac{V_3 \cdot R}{R+R}\$, which reduces to \$\frac{V_3}{2}\$.  Notice now that the units are \$\frac{Volt \cdot Ohms}{Ohms}\$, or volts, and everything is consistent.

Good catch.