[填空题]
A teacher stands well back from yec owm:v u15r+2rv:-ol2 :et :r gayzan outside dpkf5 rmuk s6an5i.x/i d21xa) oorway 0.88 m wide, and blows a whistle of frequency 750 Hz. Ignoring reflections, estimate at what angle(s) it is not possible to hear the whis a)isxfi2mxr5u1ak5/ k d6.np tle clearly on the playground outside the doorway. $^{\circ}\,$ either side of the normal.
