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# Maths - S1.3 HELP! Watch

1. How do you work out sum of x^2, when given the standard deviation, the mean and the sample size (n)?
2. (Original post by Rukia12)
How do you work out sum of x^2, when given the standard deviation, the mean and the sample size (n)?
i assume that by standard deviation you mean s in the formulae below:

if you square s then rearrange the formula on the left...

( Σ x )2/n is the same as the mean * n
3. In my textbook the SD formula is just n not n-1. When I looked at other example online they also use the n-1 one?
4. (Original post by Rukia12)
In my textbook the SD formula is just n not n-1. When I looked at other example online they also use the n-1 one?
n-1 is sample, n is population.
5. Sorry I'm quite confused :/
My question is:
A set of 15 numbers has mean 7 and standard deviation 4. A second set of 25 numbers has mean 9 and standard deviation 5. What is the standard deviation of the combined set of 40 numbers?

Do I use that equation(n-1) or the one given in my textbook(just n)?
6. Use population.
7. Okay thankyou but is why should I use population and for what type of questions should I use the sample one?
8. Use sample if question states sample, otherwise use population.

There's a rule: greek letters are population, roman are sample.

So mu is pop mean, x bar is sample mean.

Sigma is pop sd, s is sample sd.
9. Okay thank you very much all !

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