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

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.