先用汉语解释,然后用英语回答.个人观点,供参考.希望能有所帮助!
其实在国际音标(IPA,International Phonetic Alphabet)中,这两个符号代表不同的音,我们平时看到的英语音标中的/r/在国际音标中应当是[?],而在国际音标中[r]是一个颤音（trill）,这个音在英语中不存在.既然不存在,那么在英语音标中为了便于书写和印刷便采用了/r/这个记音符号用来代替国际音标中的[?].这属于宽式记音方法（broad transcription）.
有时我们会看到题目中ride的音标写成了/?aid/,那说明这是采用了严式记音方法（narrow transcription）.
回答：The sound of "ride" is sometimes transcribed as shown above because narrow transcription is employed.By broad transcription,the sound of "ride" should be /raid/.So,when referred to the same phoneme,these two symbols are just different represatations of the same sound in two transcription systems.However,in IPA(International Phonetic Alphabet),these two symbols represent two different phonemes:] is an approximant,[r] is a trill.As there is no trill [r] in English phonetics,for the sake of convenience,the symbol /r/ is commonly used to represent the phoneme ] in IPA.
