lim(θ->0)_(Cosθ-1)/θ^2 0/0 type
= lim(θ->0)_-Sinθ/(2θ) 0/0 type
= lim(θ->0)_-Cosθ/2
=-1/2
or
Cosθ=1-θ^2/2!+θ^4/4!-θ^6/6!+…
Cosθ-1=-θ^2/2!+θ^4/4!-θ^6/6!+…
(Cosθ-1)/θ^2=-1/2!+θ^2/4!-θ^4/6!+…
So
lim(θ->0)_(Cosθ-1)/θ^2
= lim(θ->0)_( -1/2!+θ^2/4!-θ^4/6!+…)
=-1/2