Graph Partial Sums of an Infinite Series on the TI84 Plus
An infinite geometric series has a sum if–1 < r < 1. But, you can use the TI84 Plus to graph partial sums. The partial sums should approach a number, which means the graph of a partial sum should have an asymptote.
Use the infinite geometric sequence:
Here are the steps to graph the partial sums of an infinite series:

Press [2nd][ZOOM], highlight TIME, and press [ENTER].

Press [Y=] to access the Y= editor.

Enter a value for nMin.
nMin is the value where n starts counting.

Press [ALPHA][WINDOW][2] to use the summation template to enter u(n).
See the first screen. Press
for n, and enter the infinite series pictured in the second screen.

Press [WINDOW] and adjust the variables.
Here are the variables changed: nMax = 20, Xmin = 0, Xmax = 20, Ymin = 0, Ymax = 2.

Press [GRAPH].

Press [TRACE] and use the rightarrow key to find the partial sums.
See the third screen.