Evaluate: limx→0 [(tan x - sin x) / (sin3 x)]
limx→0 [(tanx - sin x) / (sin3 x)] = limx→0 [(sinx (1/cosx - 1)) / (sin3 x)]
= limx→0 [(1 - cosx) / (cosx × sin2 x)]
= limx→0 [(2sin2(x/2)) / (cosx × 4sin2(x/2) cos2(x/2))]
= limx→0 [1 / (2 cos x × cos2(x/2))]
= 1/2