[填空题]
A clock pendulum oscillates at a frequency of 2.5 Hz. At it is r7td cjp3*o* xbr8ku9 m; :ktkyeleased(gok.v5 mi(cf from rest. 5 ik(cgomvf( starting at an angle of 15 $^{\circ} $ to the vertical. Ignoring friction, what will be the position (angle) of the pendulum at
(a) t = 0.25 s , $^{\circ} $
(b) t = 1.60 s , $^{\circ} $
(c) t = 500 s ? $^{\circ} $
[Hint: Do not confuse the angle of swing $\theta$ of the pendulum with the angle that appears as the argument of the cosine.]