The Student Room Group

does ± before a number

mean its standard deviation?

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Reply 1
It means it can be both positive or negative.

EG the square root of 4 is both positive and negative two.
2x2 = 4
-2x-2 = 4
Original post by asaaal
mean its standard deviation?


I think that we need a context for your question
Original post by asaaal
mean its standard deviation?


In some contexts
Standard deviation is always the positive square root of the variance.
Reply 5
You can't have negative sd.

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Reply 6
Original post by aoxa
It means it can be both positive or negative.

EG the square root of 4 is both positive and negative two.
2x2 = 4
-2x-2 = 4


(-2)^2=4, but -2 is not the (or a) square root of 4.
Original post by Slumpy
(-2)^2=4, but -2 is not the (or a) square root of 4.


Yes it is.
Original post by 19cvabn
You can't have negative sd.

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You can but the surd is complex
Original post by Black_Materia_
Standard deviation is always the positive square root of the variance.


Yes but if we use it as a measure of variance then we put a plus or minus in front.
Original post by Random1357
Yes it is.


No it isn't

the square root of 4 is 2
Original post by TenOfThem
No it isn't

the square root of 4 is 2


I'm 99% sure it's both.

If you have an explanation though I could be persuaded.
Original post by Motorbiker
I'm 99% sure it's both.

If you have an explanation though I could be persuaded.


The square root of 4 is 2 by definition

The negative square root of 4 is -2

So, 4=2\sqrt4 = 2

To include -2 you need ±4=±2\pm \sqrt4 = \pm2
Original post by TenOfThem
The square root of 4 is 2 by definition

The negative square root of 4 is -2

So, 4=2\sqrt4 = 2

To include -2 you need ±4=±2\pm \sqrt4 = \pm2


The square root of four is either plus or minus 2.

The tick notation means positive square root unless specified otherwise.

So your first line is wrong.

http://en.m.wikipedia.org/wiki/Square_root
Original post by Random1357
The square root of four is either plus or minus 2.

The tick notation means positive square root unless specified otherwise.

So your first line is wrong.

http://en.m.wikipedia.org/wiki/Square_root


Read the first line of paragraph 3 of your link. :smile:
Original post by BuryMathsTutor
Read the first line of paragraph 3 of your link. :smile:


That's exactly what I said...

It was someone else who argued that there was only 1 square root...
Original post by Random1357
That's exactly what I said...

It was someone else who argued that there was only 1 square root...


Yes but..
Original post by Random1357
That's exactly what I said...

It was someone else who argued that there was only 1 square root...


Post 2 was incorrect when it said that the square root of 4 was both positive and negative 2

The square root of 4 is 2

The negative square root of 4 is -2



Your link supports this definition
The square root of 4 is 2.

The negative square root of 4 is -2. The square root and negative square root are not the same thing.

The solutions to y^2=4 are y=2 or y=-2.

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noun
noun: square root; plural noun: square roots
a number which produces a specified quantity when multiplied by itself.





-2 satisfies this for 4 thus both 2 and -2 are square roots.

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