By Tech Powered Dad | September 30, 2011

Got an uncharacteristic number of hits on the site yesterday for the search “TI-36X Pro” yesterday and couldn’t figure out why until I saw a post on another site and a Youtube video about a “bug” in the TI-36X Pro. I’m not going to post the links to those sites in part because the author of the complaint has needlessly chosen to remain anonymous and because the article itself never explains the error and is confusing a lot of people. The Youtube video does explain the complaint in more detail, however, and I’d like to address the so called “bug.”

The calculation in question is presented as an area of a circle problem. Specifically, use the TI-36X Pro to do this equation:

pi*12.5^2

The calculator will return the result 490.8738521, a correct approximation of the solution.

Now, use the toggle solution button above the enter button to change the result to an exact solution. You will see the answer (625 pi)/4. This is still correct. However, the issue arises when you then go into the math menu to get a mixed number results, option 1.

If you do this, you’ll get the result, 156 pi/4. The author of the video goes on to say this is “off by a factor of 4 almost.” He goes on to demonstrate another example which he says is off by a factor of around 3.

Depends on how you interpret the result. If you interpret 156 pi/4 to mean 156 times pi/4, then yes, this is an incorrect solution. The reason is that by 156 pi/4, the calculator is trying to communicate 156 and ^{1}⁄_{4} pi.

While I agree that I would prefer to see 156 and ^{1}⁄_{4} pi as the displayed result, this is not a calculation error by the TI-36X Pro. In fact, if you take the calculator’s mixed number solution and multiply by 4, you will get 625 pi, the correct solution. This means that while the display may be confusing, the calculator is internally consistent in how it is handling the calculation.

It’s easy to see from the comments on those other sites that the author of the complaint has succeeded in convincing a lot of people that the calculator is simply spitting out randomly incorrect numbers. This is clearly not the case. While he claims ignorance as to what the error is on his video, I’m skeptical that anyone who would be capable of understanding the mathematics behind this problem would be incapable of figuring out what the calculator is actually displaying. And while it’s no excuse for the misleading display of the problem, I’m stumped as to why you’d want a mixed number with pi anyway. I expect my students to write (5 pi) / 2, not 2 and ^{1}⁄_{2} pi.

In any regard, this doesn’t affect my positive recommendation of the TI-36x Pro. While I’d like to see the way these results displayed in a less ambiguous manner in a future release, the suggestion that the calculator is miscalculating is incorrect.