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by JesseAldridge
2878 days ago
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I think a lot of people need to start from the basics because they don't have a good foundation in math. The core problem is schools will push you along if you can somehow produce the correct answer for 70% of the problems on a test. Combine this with intense pressure not to fail and you will very likely end up in higher level math courses with many gaping holes in your foundational knowledge. You thus end up relying on tricks and memorization rather than useful understanding. Here is a TED talk where Sal Khan of Khan Academy talks about this: https://www.youtube.com/watch?v=-MTRxRO5SRA After struggling to understand advanced math in a lot different contexts I decided to go through the entire K-12 set of exercises on Khan Academy. I blazed through the truly elementary stuff like counting and addition in a few hours, but I was suprised at how quickly my progress started slowing down. I found I could not solve problems involving negative numbers with 100% accuracy. Like (5 + (-6) - 4). I would get them right probably 90% of the time but the thing is Khan Academy doesn't grant you the mastery tag unless you get them right 100% of the time. I found most of my problems were due to sloppy mental models. Like, I didn't understand how division works -- if someone were to ask me what (3/4) / (5/6) even means conceptually I would not have been able to provide a coherent, accurate explanation. "Uh... it's like taking 5/6 of 3/4... wait no that's multiplication... you need to flip the second fraction over... for some reason..." It was around the 8th grade level that I found myself having to actually work hard. (What does Pi even mean?) And I've been through advanced Calculus courses at the university level. |
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In case you (or others reading this) still struggle to formalize division, a very nice way to conceptualize it is as the inverse of multiplication. This neatly sidesteps the problem of trying to figure out a clean analogue for what it means to to multiply a fraction of something by another fraction of something, since the intuitive group-adding idea of multiplication sort of breaks down with ratios.
Addition is a straightforward operation, but subtraction is trickier. For all real x there exists an additive inverse -x satisfying x + (-x) = 0. So to subtract 3 from 4 we instead take the sum 4 + (-3) = 1.
Likewise to multiply 3 by 4 we add four groups of 3: 3 + 3 + 3 + 3 = 12. We accomplish division by using a multiplicative inverse: for all real x there exists a 1/x such that x(1/x) = 1.
So (3/4) / (5/6) is equal to (3 * 1/4) / (5 * 1/6). In other words, take the multiplicative inverse of 4 and 6 and multiply them by 3 and 5 respectively. Then multiply the first product by the inverse of the second product.
This is the axiomatic basis of division as "repeated subtraction": subtraction is the sum of a number and another number's additive inverse, and multiplication is repeated addition. Then division is the product of a number and another number's multiplicative inverse. From this perspective you need not even understand division computationally if all you'll ever deal with are fractions and not decimals.