this post was submitted on 19 Jul 2023
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Multiplication is a notation which means add some number by itself a number of times.
5 x 3 = 5 +5 + 5
2 * 4 = 2 + 2 + 2 +2
So when you see some like 2 + 4 * 2 it literally means. 2+4+4
By that logic it could just as well be 2 + 4 * 2 = (2 + 4) + (2 + 4) = 12. You still need to know to multiply first, or it's arbitrary
Edit: a lot of you are missing my point. The expression above is wrong, duh, but my point is that the choice to "expand out" the multiplication first is a convention that the mathematics community agreed on, not a fact that can be proven or measured. That's why it's arbitrary. @kogasa put nicely, PEMDAS is just a notation, it's how we agreed to read and write our math, but the underlying math is no different. If we all agreed to scramble the order of operations, say to add before we multiply, expressions will look different, parentheses may need to be added or removed, but they will still be mathematically consistent if we are consistent in writing and reading in that agreed upon order of operations.
No you expand it all out first.
I know that my example is wrong, I'm trying to make a point
In your example you lose distributivity. (2+4)2 is 22+4*2, which doesn't matter for numbers but it matters for algebra. If addition comes first then there's no way to represent distribution.
The distributive law, assuming commutativity and other axioms, is a*(b+c) = (a*b) + (a*c). Notice how it does not matter in which order you evaluate + and * in this expression due to my use of parentheses.
PEMDAS is notation. It has no influence on the actual underlying math, only how we write it.
You're absolutely right, not sure what I was thinking.
Thanks, I've been trying to figure out how to put this and you did it concisely!
It's not logic, it's that what it means. 2*4 literally means 2+2+2+2. Just like 8/2 means how many times you can add 2 to itself until you get 8.
That's what it means.
So why use braces? Because in more advanced maths you have more complex expressions that can't be express in just multiplication which often occur in algebra or beyond.
For example what does 2 * ( a + 3) actually mean? Like why do we need to do the addition first. Its because we don't know how many times we need to repeat the addition until we know what a means.
Let's say a are points on an axis, and at some point it is worth three the. At that moment that expression is 2 * (3 +3) = 2 * 6 which is equal to 2+2+2+2+2
But in the next moment a might be 1
Right?
What's arbitrary are the labels on the rules. The rules themselves aren't arbitrary.
See my edit, I think you misunderstand me
Order of operations proof - simple version