The cross product on the right-hand side of this expression is a surface normal determined by the parametrization.
ð ə WORD_BOUNDARY k ɹ ɔ s WORD_BOUNDARY p ɹ ɑ d ʌ k t WORD_BOUNDARY ɔ n ð ə WORD_BOUNDARY ɹ aɪ t h æ n d WORD_BOUNDARY s aɪ d WORD_BOUNDARY ʌ v WORD_BOUNDARY ð ɪ s WORD_BOUNDARY ɛ k s p ɹ ɛ ʃ ə n WORD_BOUNDARY ɪ z WORD_BOUNDARY ʌ WORD_BOUNDARY s ɜː f ɪ s WORD_BOUNDARY n ɔ ɹ m ə l WORD_BOUNDARY d ɪ t ɜː m ɪ n d WORD_BOUNDARY b aɪ WORD_BOUNDARY ð ə WORD_BOUNDARY p æ ɹ ə m ɛ t ɹ ɪ z eɪ ʃ ə n WORD_BOUNDARY