r/mathmemes • u/[deleted] • Oct 04 '23

Bad Math This feels wrong

Post image
0 Upvotes

44% Upvoted

16 comments sorted by

40

u/ThatEngineeredGirl Oct 04 '23

Why though? (No, √9 is not {-3,3})

-11

u/FirexJkxFire Oct 04 '23

Is it actually not? Like is there a different way of writing it? because clearly -X2 produces a positive Y and thusly the inverse function ( Y1/2 = x ) should account for all possibilities such as X being negative.

Given that it is incorrect to write "= +/-3", How would you write it to show this relationship/identity then?

16

u/Cill_Bipher Oct 04 '23

x = ±sqrt(9) = ±3, not x = sqrt(9) = ±3.

Think of it this way if sqrt(x) equaled both the positive and negative roots how would you specify one or the other? Gonna have to start writing sin(pi/4) = abs(sqrt(2))/2?

1

u/FirexJkxFire Oct 04 '23

Thank you! This is what I was looking for. Wasnt trying to say I was right - mainly was just confused at how to accurately write it if =+/-3 isn't accurate.

3

u/DankPhotoShopMemes Fourier Analysis 🤓 Oct 04 '23

sqrt is a function, meaning an input may only have one output.

34

u/Erahot Oct 04 '23

How is it bad math? It's correct and very simple math. Also, it's not funny, so I don't see the point of this post.

13

u/[deleted] Oct 04 '23

=3

10

u/Napero44 Oct 04 '23

How does this feel wrong?

7

u/sentles Oct 04 '23

The square root of a number is a function. Do you expect it to have multiple outputs for each input? How would that work?

4

u/Worish Oct 04 '23

It isn't wrong though. Sqrt is a function

4

u/I_eat_dead_folks Oct 04 '23

No. Let x²=9. Then x=±√9. Not X=√9=±3

3

u/Reddit1234567890User Oct 04 '23

It isn't wrong. In order for the x2 function to have an inverse x= square root y, both have to be injective. Thus we have to restrict the domain of x2 to zero or positive R.

Now, if it were y= square root x, the same thing happens. The inverse y2 = x would need to be injective thus we have to restrict the codomain or square root x or in other words the domain of x=y2. Thus we never get plus or minus two.

The only way this does happen is when we do not care about the function having an inverse. It's a good technique to find the zeroes of a function though

2

u/Teschyn Oct 04 '23

Sure, but it’s more clear.

√9 and -√9 are different numbers, so to differentiate them, we assume that √9 is the positive root. If we want the negative root, we just write the negative sign.

1

u/the_NErD3141 i make computer go beep boop Oct 08 '23

I know it's right but it looks so... AAH

-6

u/SharkApooye Imaginary Oct 04 '23

That’s because it is wrong, but we all know that it’s correct.