Then, we did the following "by hand".
I asked my students, "what would Desomos do with this problem". I wasn't really sure. When I entered the expression, it didn't give me "an answer", it gave me a graph. "Why is it doing that?" I wondered.
And then it dawned on me! Desmos was graphing the polynomial that resulted from the summation. We confirmed this as a class by entering the expression 1 + x + x2/2 + x3/6 + x4/24 in line 2 and it graphed over the graph that was already there. As I was thinking aloud and having my lightbulb moment, my students asked me, "Wait, you mean you haven't done this in Desmos before?" They thought I was acting surprised and had done this before. I do that sometimes. But not this time, my surprise was real. About an hour later, I shared my discovery with the colleagues in my math department. They were also impressed. This is definitely something that can't happen on a TI-whatever.