√（20％－11％）^2×0.5 +（10％－11％）^2×0.3 +（－10％－11％）^2×0.2=11.36

（20％－11％）^2×0.5
=(0.2-0.11)^2×0.5
=0.09^2×0.5
=0.0081×0.5
=0.00405
（10％－11％）^2×0.3
=(0.1-0.11)^2×0.3
=(-0.01)^2×0.3
=0.0001×0.3
=0.00003
（－10％－11％）^2×0.2
=(-0.1-0.11)^2×0.2
=(-0.21)^2×0.2
=0.0441×0.2
=0.00882
所以跟号内=0.00405+0.00003+0.00882
=0.0129
开根号,√0.0129约等于0.1136=11.36%