trigo

AC / AE = sin∠AEC
AC / 1 = = sin(π - (α + β))
AC = sin(α + β) ,DE = sin β , BE = DE / sin α
BE = sin β / sin α ,BD = (cos α) BE
BD = sin β cos α / sin α ,AD = cos β
We have sin α = AC / (AD + BD) = DE / BE sin(α + β) / (cos β + sin β cos α / sin α) = sin β / (sin β / sin α)sin(α + β) = (cos β + sin β cos α / sin α) sin β / (sin β / sin α)sin(α + β) = sin α cos β + sin β cos α


2012-04-26 00:20:58 補充：
不用理會第二行 : AC / 1 = = sin(π - (α + β))