[填空题]
Francesca, who likes givhd r87cc.o:y )n;p dl;)k pphysics experiments, dangee o llb1ry)*.les her watch from a thin piece of string while thol*l1ey.bre ) e jetliner she is in takes off from JFK Airport (Fig. 4-58). She notices that the string makes an angle of 25$^{\circ}$ with respect to the vertical as the aircraft accelerates for takeoff, which takes about 18 s . Estimate the takeoff speed of the aircraft.
