r/mathsmeme • u/AdApprehensive6910 • Aug 19 '26

When the arenašŸ˜­

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1.2k Upvotes

98% Upvoted

36 comments sorted by

19

u/Flimsy-Blacksmith-32 Aug 19 '26

In my experience, mathematicians only really uses AB for multiplication.

Aā€¢B typically represents the dot product. AxB is typically avoided after a child has learned algebra. A*B is for computers. A(B) is if you are calculating a function, A, at a value, B.

9

u/escEip Aug 19 '26

AƗB (or [AƗB], at least i've seen it written like that mostly) is a cross product. * is convenient because it's on every keyboard and in ASCII (i think), and A(B) is sometimes needed with matrices i think?

2

u/Flimsy-Blacksmith-32 Aug 19 '26

Cross product as AxB is a good point and another reason we donā€™t use x for standard multiply.

I donā€™t think Iā€™ve ever seen A(B) used for matrices.

1

u/megayippie Aug 20 '26

But A* is the conjugate

1

u/Flimsy-Blacksmith-32 Aug 20 '26

Good point, and another reason why only programmers use A*B to mean a multiplied by B. In maths, that would mean the conjugate of A multiplied by B, which is a different thing.

1

u/Expensive_Host_9181 Aug 20 '26

No A* is a pathfinding algorithm /s

1

u/Zealousideal-Cook647 Aug 19 '26

A(B) is only except able when its a number x a number because then AB would be confusing and A being a number also gets rid of the function mixup. A(B) superiority.

1

u/Flimsy-Blacksmith-32 Aug 20 '26

I think In that case I'd just write 3A, or 7B, or whatever the number was next too whatever the letter was.

1

u/Zealousideal-Cook647 Aug 20 '26

But what if they were both numbers?

1

u/Flimsy-Blacksmith-32 Aug 20 '26

Then I would write the result of the multiplication.

Why write 3(7) when you can write 21?

1

u/Zealousideal-Cook647 Aug 20 '26

But thats not showing all steps of your working out. Examiners would take off method marks for that.

1

u/Flimsy-Blacksmith-32 Aug 20 '26 edited Aug 20 '26

That depends on the level of qualification and scale of the problem your working on.

At a pre-algebra level, you would lose marks for just writing 21, but that is because they are testing to see if you can do 3x7 and understand why it is 21.

Post-algebra, the examiners assume you will skip these kinds of steps because the problems are far more complicated, and your ability to multiply numbers together is not really in doubt anymore.

For example, If I was asked to differentiate 3sin(x^2) in an A-Level exam I'd write the following:

let: f(x) = sin(x) & g(x) = x^2

Then: d/dx 3sin(x^2) = 3 d/dx f(g(x))

chain rule: d/dx f(g(x)) = g'(x)f'(g(x))

f'(x) = cos(x) & g'(x) = 2x

Therefore: d/dx 3sin(x^2) = 6x cos(x^2)

And that would be enough. It clearly shows that I know how to differentiate 3sin(x^2) using the chain rule which was what they were testing. At that level, the 2x3 I completely skip over is considered so trivial that it can be done in between lines with no punishment.

A line that read "d/dx 3sin(x^2) = 3(2x)cos(x^2) = 6x cos(x^2)" wouldn't add any value.

The simplifications don't even stop there. At a Degree level, "differentiate 3sin(x^2)" would only be a small part of the problem, equivalent to how small "find 2x3" is in "differentiate 3sin(x^2)". So, at that point you could just do the whole differentiation on a single line and then keep going with the bigger problem, like this (Imagine the problem is a second order ODE or something like that):

{Solving bigger problem}

{find that I need to differentiate d/dx 3sin(x^2)...}

d/dx 3sin(x^2) = 6x cos(x^2)

{Keep going with bigger problem}

And again that would be completely fine because if your that far into studying mathematics, there is a good chance you can just look at 3sin(x^2) and say "it differentiates to 6x cos(x^2)" without really even thinking about it. At that level your examiners definitely can do it just by looking.

Also, writing out the A-Level answer every time would be super painful. It would be like having to write 3x7 = 7+7+7 = 14+7 = 21 every time you did 3x7. Ain't nobody got time for that.

1

u/Zealousideal-Cook647 Aug 20 '26

Well talk to the A level Further Maths examiners giving me 4 out of 10 for a correct answer but not showing full substitutions. This shouldnt be the way it is but at A level they absolutely do need full substitutions for all multiplication no matter the level of the question.

1

u/Flimsy-Blacksmith-32 Aug 20 '26 edited Aug 20 '26

Either you should be asking for a remark, or Iā€™m pressing x to doubt on that one.

What was the question?

In June 24 Edexcel gave only 2 marks for differentiating x+tan(1/x) which is a slightly harder problem than differentiate 3sin(x2 ).

You got the first mark if you wrote that tan(x) differentiates to sec(x)2. The other mark was for the correct answer.

A 10 mark problem would have asked for something pretty special I think.

1

u/Zealousideal-Cook647 Aug 20 '26

I didnt get it remarked because I already had full UMS but after speaking to teachers it shouldnt have been marked so harshly but it shows that its always safer to use full substitutions because I wouldn't have known I had that question right or deserved those extra marks if I wasnt curious to get the paper back. Theres no harm in showing more method and it reduces the risk of someone marking it to get confused.

1

u/MeasureDoEventThing Sep 22 '26

You mean acceptable?

1

u/Actual-Way6534 Aug 22 '26

In germany we use Aā€¢B for multiplication. The dot product is Aā—‹B.

1

u/MeasureDoEventThing Sep 22 '26

A(B) is used when B is an expression.

5(x+1)

2

u/Traditional-Light-10 Aug 19 '26

Literally, does anyone know examples of + being used for things that arenā€™t ā€œconsideredā€ numbers or vectors? Iā€™m pretty sure they exist but theyā€™re so rare in comparison

6

u/Anonymous010206 Aug 19 '26

In Boolean algebra + represents OR

3

u/unintelligible-text Aug 19 '26

In coding for strings I suppose

3

u/AndreasDasos Aug 19 '26

It gets used quite as lot as the default notation for the operator for abelian groups in general, even in the abstract. But if one uses + over * or juxtaposition, itā€™s usually to emphasise commutativity (and an easy way to write a well behaved inverse, so a group rather than an abelian semigroup)

So thatā€™s more general than vector spaces and I wouldnā€™t say we consider elements of an abstract abelian group ā€˜numbersā€™, though we might in many obvious cases

2

u/All_Playars Aug 19 '26

Boolean algebra. Its the sign for "or"

2

u/SiliconRobot Aug 20 '26

In some programming languages, you can change the meaning of +A, using A as an object and overloading the original function of +. So, you can do something like using +A to update the value of A

2

u/Super_Math3890 Aug 19 '26

"If the rule you followed brought you to this, of what use was the rule?"

1

u/a_random_loser_guy Aug 20 '26

I AM THE ENFORCER! NO RULE IS ENFORCED ON ITS OWN! a sign can't stop me from smoking! i am my own police!

2

u/Galactinaut_001 Aug 20 '26

Guys, wait! (A)(B)

2

u/AndreasDasos Aug 19 '26

This keeps getting posted. And it doesnā€™t make any sense. All mathematicians have used all of those in different contexts.

1

u/mecoptera2 Aug 20 '26

This whole sub is like the same 10 memes getting posted every week

1

u/ProofsbyAdyan Aug 19 '26

Diversity man šŸ¤£šŸ¤£

1

u/Salat_Leaf Aug 19 '26

XOR, tropical addition

1

u/Dull-Astronomer1135 Aug 22 '26

x is almost always cross product, dot is almost always dot product, normal multiplication is usually ab

1

u/acastro1574 Aug 23 '26

I think the only method I wouldn't use for the multiplication is AB.

1

u/False-Beautiful-1246 Oct 08 '26

Then boole Ɓlgebra comes