draw and recognise a summing opamp amplifier circuit and describe its applications;
use the formula V_{out} = – R_{f} ( V_{1} + V_{2} + V_{3} )
R_{1} R_{2} R_{3}
recall that the input resistance of each input is equal to the value of its input resistor, and that the circuit has a virtual earth point;
What does this circuit remind you of? 
What does this mean in simple terms? 

Explain why the statement V_{out} = (V_{1} + V_{2} + V_{3}) is true. 

Suppose V = 5 volts and R = 10 k and R_{f}_{ }= 1k. Use these to work out V_{out} for the binary word 1101. 
Summary

Links 

Electronics Tutorials 

WISConline presentation 

Texas Instruments opamp collection 

Video tutorial 