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If mean of squares of deviations of a set of n observations about -2 and 2 are 18 and 10 respectively, then standard deviation of this set of observations is

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`sum(x_i-22)^2/N`
`sum(x_i+2)^2/N=18-(1)`
`sum(x_i-2)^2/N=10-(2)`
`sum(x_i^2+4+4x_i)=18N`
`sumx_i^2+sum4+4sumx_i=18N`
`sumx_i^2+4sumx_i=14N-(3)`
`sumx_1^2+4N-4sumx_i=10N`
`sumx_i^2-4sumx_i=6N-(4)`
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