 So now that we've looked at simple circuits for resistors, as well as series circuits and parallel circuits, we can start to tackle combination circuits. So quick reminder, a basic resistor circuit has just one battery and one resistor. And it's governed by Ohm's law. And we can use the different algebraic rearrangements for Ohm's law. So for a series circuit, there is one path around the whole circuit. And it never splits. So that's sort of our key thing we're looking for. In a parallel circuit, there's two paths around the circuit, one going through resistor 1 and one going through resistor 2. But the key that we're looking here is at some point, in this case up here, it splits off. And then those two paths come back together at a second point. So in combo circuits, we're going to have part series and part parallel. In the series section, there's one path with no splits. For the parallel section, it splits and comes back together. And we could also think of this in terms of that each side before it splits, I've got the same high side potential. And then when it comes back together, I'm going to have the same low side potential for both paths. So let's look at an example here. In this case, we can look at this section marked off by these two little purple dots. And between them, I've got a single path that goes through R1 and R2. And so this is a series section. But I don't have one path that goes through all the resistors, so R3 is going to have to be treated separately. But first, I can deal with R1 and R2 as a series section of the circuit. Here's another example. And here, if I look between these two purple dots, I've got one path and another path. And it splits apart at one place, comes back together at the other place. So this is a parallel section, where now R2 and R3 are in parallel. R1's not in parallel with them, so we'd have to come back and treat this part separately. Now let's look at a third example. A lot of students will glance at this really quickly, look at these two dots, and say, oh, look, R1 and R2 are in series, but they're not. See, there's a point in here in the middle where it's going to split off. So this is not a series section. Instead, we want to look at these two points. And these two points, I've got one path and a second path. So it split off at this point, and it came back together at this point, so this is a parallel section. Now let's look at this section. Now, if I look at these two points here, I might be tempted to say, oh, look, R1 and R3 are parallel to each other. See, that wire and that wire is parallel. But if I actually start trying to analyze these parallel sections, I've got one path down here. My second path, though, doesn't include just R1. It's got R1 and R2 on it. So I can't treat R1 being in parallel with R3 or R2 in parallel with R3. First, I'm going to want to deal with R1 and R2. So instead, if I look at these two points, I can clearly see that I've got one path between those two points. And so this is a series section for R1 and R2. Once I've dealt with R1 and R2 in series, then I could come back and look at R3. In general, when you're dealing with these circuits, you first have to recognize the series and parallel sections. Then you can replace those small sections with the equivalent resistance for that section, either in series or in parallel. Then you mathematically work through the full circuit. This video, we really emphasize this recognizing what sections are in series and parallel, because if you don't do that right, nothing else is going to work out for your circuit. So that introduces combo circuits. You'll need a lot more practice and a look at those other strategy steps.