Web plots are a really interesting representation of the behavior of a recursive sequence, and you can use the TI-84 Plus to graph web plots. Web plots actually look a little like spider webs. Following are a few ground rules when working with web plots:

  • Sequence must be recursive: The sequence is required to have one level of recursion u(n–1); using u(n–2) is not allowed.

  • Sequence cannot reference n directly.

  • Sequence cannot reference any defined sequence except itself.

Here, you use the recursive sequence, an = –1/2(an–1), where a1 = 8.

Here are the steps to enter and graph a web plot in your calculator:

  1. Press [2nd][ZOOM] to access the Format menu.

  2. Use the arrow keys to navigate your cursor to Web in the top row and press [ENTER].

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

  4. Enter the recursive formula for u(n).

    Use u(n–1) for the previous term.

  5. Enter u(nMin).

    Enter the initial term in the sequence as shown in the first screen.

  6. Press [ZOOM][6].

    If you aren't sure what the graph of the web plot will look like, a ZStandard graphing window is a good place to start, as shown in the second screen.

    The line graphed at y = x is not part of the web plot; it is called the reference line.

  7. Press [TRACE] and press the right-arrow key repeatedly to draw the web.

    See the third screen. The web plot helps you see that this sequence converges.

    To clear a webplot, press [2nd][PRGM][ENTER].