Differentiation under the integral sign

http://en.wikipedia.org/wiki/Differentiation_under_the_integral_sign

原來有好多人都唔知呢條式嘅存在...，anyway 我都 state 左條 formula 同 condition 俾班同學聽，唔知佢地會唔會唔識證 (正常知條 formula 嘅詁 year 1 知識就夠哂數=.=)。