A data consists of n observations: x_1, x_2, …., x_n. If Σ_i=1^n (x_i+1)^2=9 n and Σ_i=1^n (x_i-1)^2=5 n, then the standard deviation of this data is:

A data consists of n observations : $\displaystyle x_1, x_2, \ldots ., x_{\mathrm{n}}$. If $\displaystyle \sum_{i=1}^{\mathrm{n}}\left(x_i+1\right)^2=9 \mathrm{n}$ and $\displaystyle \sum_{i=1}^{\mathrm{n}}\left(x_i-1\right)^2=5 \mathrm{n}$, then the standard deviation of this data is :