证
F(xy,uv) = ln(xy) ⋅ ln(uv) = (ln x + ln y) ⋅ (ln u + ln v)
= ln x ⋅ ln u + ln x ⋅ ln v + ln y ⋅ ln u + ln y ⋅ ln v
= F(x,u) + F(x,v) + F( y,u) + F( y,v) .