（X1+2,X2+2,…Xn+2)/n=10 (x1+x2+...+xn)=10n-2n=8n
((x1+2-10)²+(x2+2-10)²+.+(xn+2-10)²)/n=3 (x1+2-10)²+(x2+2-10)²+.+(xn+2-10)²=3n
2(x1+x2+...+xn)=16n 2(x1+3/2+x2+3/2+...+xn+3/x)/n=19
((2x1+3-19)²+(2x2+3-19)²+.+(2xn+3-19)²)/n
=(4(x1-8)²+4(x2-8)²+...+4(xn-8)²)/n
=4[(x1-8)²+(x2-8)²+...+(xn-8)²]/n
=12 标准差=根号下12=2根号下3