>>980

f(x) = 1/{σ√2π)} exp［-(x-m)^2/ 2σ^2],

下位30%　････　偏差値(m-0.5244σ)以下

f(x)

ma = ∫[-∞, m-0.5244σ] x･f(x) dx } / ∫[-∞, m-0.5244σ] f(x) dx{
　= m + ∫[-∞, m-0.5244σ] (x-m) f(x) dx / 0.3
　= m + (σ/√2π)∫[-∞, -0.5244] t･exp(-tt/2) dt / 0.3
　= m + (σ/√2π) [ -exp(-tt/2) ](t=-∞, -0.5244) / 0.3
　= m + (σ/√2π) [ -0.8715 / 0.3 ]
　= m - 1.159σ
　= m - 11.59