[填空题]
A teacher stands well back from an outside doorway 0.88 m wide, 31jv.u 9yxj1 . 3.p bv ivacy)ct)djwdand blowvfz3 +)zi-ni 8h0pwhls a whistle of frequency 750 Hz. Ignoring reflections, estimate at what angle(s) it is not possible to hear the whistle clearly on the playground outside the do8hp )z3vli+w n0f-zhi orway. $^{\circ}\,$ either side of the normal.
