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I started the “potential due to two charges” activity before doing the power series activity. My goal was to provide a little more physical motivation for power series, and to break up the math (we just did “distance between two points”) with some physics. I just had students write down a general expression for the potential due to $+Q$ charges $\pm D$ from the origin on the $x$-axis.

As part of the wrap up for this activity, I remind students of the small angle approximation $\sin(\theta)=\theta$. I ask them what this has to do with power series expansions. This is a nice chance for them to see that something familiar, which they may never have realized the origin of, comes from the work we've just been doing. This leads nicely into a discussion of how small does a “small angle” have to be.