Solution:

If input voltage sources and resistors are connected to the noninverting terminal as shown in fig $(a)$, the circuit can be used either as a summing or averaging amplifier through a selection of appropriate values of resistors, that is $R_1$ and $R_F$.

To verify the function of the circuit, the expression for the output voltage must be Obtained.

The input resistance $R_{\text {if }}$ of the noninverting amplifier is very large.

Therefore, using the superposition theorem the voltage v, at the noninverting terminal is,

$ \begin{aligned} &V_1=\frac{R / 2}{R+R / 2} V_a+\frac{R / 2}{R+R / 2} V_b+\frac{R / 2}{R+R / 2} V_c \\\\ &V_1=\frac{V_a}{3}+\frac{V b}{3}+\frac{V_c}{3} \\\\ &V_1=\frac{V_a+V b+V c}{3}\\ \end{aligned}\\ $

Hence, the output voltage $V_0$ is,

$ \begin{aligned} V_0 &=\left[1+\frac{R_f}{R_1}\right] V_1 \\ V_0 &=\left[1+\frac{R_f}{R_1}\right] \frac{V_a+V_b+V_c}{3} \end{aligned} $