[填空题]
A certain dog whistle operates at 23.5 kHz, whilua n0-*boeym .tszl 4a8z-ro.i+f 9rre another (brand X) operates at ozaun- fbrlr9 tm4.- zrao .y+esi 8*01ulex2n-hgz2y 0.dwean unknown frequency. If neither whistle can be heard by humans when played separately, but a shrill whine of frequency 5000 Hz occurs when they are played simultaneously, estimate the operating freewl2d1zxg-0eyn2. uhquency of brand X. kHz
