Inverting Summing Amplifier

Governed by:

$$V_{\text{out}}=-\sum \left(V_{i{n}_i}\times \frac{R_f}{R_{i_1}}\right)$$

If all $R_i$ are equal, we have:

$$V_{out}=-\frac{R_f}{R_i}\cdot \sum V_{i{n}_i}$$

There is also a non-inverting version of this.