Graph Partial Sums of an Infinite Series on the TI-84 Plus

By Jeff McCalla, C. C. Edwards

An infinite geometric series has a sum if–1 < r < 1. But, you can use the TI-84 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:

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

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

  3. Enter a value for nMin.

    nMin is the value where n starts counting.

  4. 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.


  5. Press [WINDOW] and adjust the variables.

    Here are the variables changed: nMax = 20, Xmin = 0, Xmax = 20, Ymin = 0, Ymax = 2.

  6. Press [GRAPH].

  7. Press [TRACE] and use the right-arrow key to find the partial sums.

    See the third screen.