This really boggled my mind! (algebra)

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So, I'm not a mathematician, didn't go past highschool. I did think I was ok with algebra at least. I paused the video when the question was shown. My first answer was 3, but I put it on paper to check myself. Yep. It's still 3. Since circumference is πD, circle B has 3 times the diameter of circle A, therefore circle A has to make 3 revolutions around circle B, Wrong! It never registered with me that I had to add the radius of circle A into the calculations, effectively increasing the circumference of circle B. Some of you will have your eyes glaze over, that's ok. I'm sure someone here will say "You bonehead, this is obvious." Most folks don't use algebra/trig/geometry, etc. after they leave school. I don't use math for many practical applications, but I do find it interesting.
I'm just enough of a nerd that I'll try to solve equations in my head when I'm getting sleepy while driving to keep me alert. 
I had to finish the video to grasp the concept.

Feel free to put me on ignore!
(This post was last modified: 03-06-2024, 07:18 PM by tommag.)
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Just one question... what video?
(03-06-2024, 11:35 PM)olfart Wrote: Just one question... what video?
Oops!
https://youtu.be/FUHkTs-Ipfg?si=ByDUEZm7PHyJw5Pi
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I disagree with the presenter's results. The question, as asked, was "At the end of how many revolutions will the center of Circle A reach its starting point?" The center of the circle actually has no bearing on the results, as the entire circle A will travel around circle B 3 times in order for circle A's center to return to the starting point. Therefore I call BS on the entire premise of the video, which was to make the presenter appear smarter than the test writers.
I have seen this before and using a fixed mark on the smaller circle as an up reference and counting revolutions so the fixed mark is orientated up in order to call it a revolution is flawed.

The wording of the question is the test. You must read it the way the writer wants it to be read.

There is the physical rotation and the visual rotation. If you straightened out the circles into lines the answer is three but because the tracking goes around a circle the visual rotation does not make a full rotation.
(This post was last modified: 03-07-2024, 10:29 PM by Towtruck.)



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