hˈɪɹ ðə fˈaɪnəl iːkwˈɔlᵻɾi fˈɑːloʊz baɪ ðə ɡɹˈeɪdiənt θˈiəɹəm, sˈɪns ðə fˈʌŋkʃən ɪz dˈɪfɚɹˌɛnʃɪəbəl ˈɔn ɪf .
Here the final equality follows by the gradient theorem, since the function  is differentiable on  if .