My textbook has this excerpt: I am not sure why it refers to Example 10.6 as that is unrelated. I don't understand to be honest. How can you calculate the Thevenin equivalent at different frequencies? Let us say that you have two voltage source: v1(t)=10cos(5t) and v2(t)=10cos(2t). So how do you calculate the Thevenin equivalent at the first frequency of 5 Hz? Do you change v2's frequency to 5 Hz?