More Adventures in Innumeracy

MathLunacyWriting error-free books is difficult. I get that. But every once in a while certain errors just rub me so wrong I just have to scream. My latest encounter is with Primo’s pre-algebra student workbook (Horizon’s Lesson #95). Click on the picture to be “amazed.”

My first reaction was, “In whaaaat universe?”

In the current lesson the cones are presented as right circular cones, meaning the length of the side relative to the radius and height is governed by good ‘ol Pythagoras.

Thankfully, Primo has been paying attention and recognized that the “slant height” (i.e., the side of these right circular cones) cannot be the same length as the height. He wrote the word “impossible” next to each of the four problems that were simply presented incorrectly. Undaunted, Primo just decided to go through the motions as if it were a archetypal skills-test and executed in accordance with the instructions; then, we made him compute the correct answer. He’s explaining the impossibility to Secondo as I type this.

I see what happened. The table from Lesson #94 (on right circular cylinders and oblique cylinders) was copied to Lesson #95. This is not a problem for cylinders: if height and “slant height” are the same, then it must be a right cylinder.

MathLunacy 1However, what really chaps my hide was what I found in the answer key. Occasionally, the answer key is wrong. Again, I get it; it is not easy to make an error free book. We use the key just to get through the checks quickly. When there is a discrepancy between the key and Primo’s work, GranolaGirl works it out herself to see who’s right; but if you look at this picture of the answer key for this lesson I come away with a startling realization.

The folks putting together the answer key just blindly plugged in the numbers to arrive at the incorrect answer, never recognizing what should have been painfully obvious. Remember, the answers in this teachers guide were not automatically generated; someone had to “plug-n-chug” the answers.

What baffles me is how anyone in the “biz” could miss such an obvious error.

imagesPythagoras: “Congratulations, sir. All your astronauts have died due to a construction error related to a misunderstanding of basic geometry. Maybe you should wear that cone on your head.”



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