If the frequency ,mean and standard deviation of two sets of observations be denoted by and respectively, then prove that where, and are the mean and standard deviation of the combined set of observations.

thanks in advance!

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- December 4th 2012, 11:32 PMearthboyeasy standard deviation problem
If the frequency ,mean and standard deviation of two sets of observations be denoted by and respectively, then prove that where, and are the mean and standard deviation of the combined set of observations.

thanks in advance! - December 5th 2012, 05:34 PMchiroRe: easy standard deviation problem
Hey earthboy.

Do you know results regarding variances for random variables?

(Hint: things like Var[aX + bY] = a^2*Var[X] + b^2*Var[Y] if X and Y are independent)