https://s0.wp.com/latex.php?latex=%5Cdisplaystyle+%5Cfrac+%7Bd%7D%7Bdx%7D%5Cleft%28%5Cint+_%7Ba%28x%29%7D%5E%7Bb%28x%29%7Df%28x%2Ct%29%5C%2Cdt%5Cright%29%3Df%5Cbig+%28x%2Cb%28x%29%5Cbig+%29b%27%28x%29-f%5Cbig+%28x%2Ca%28x%29%5Cbig%29+a%27%28x%29%2B%5Cint+_%7Ba%28x%29%7D%5E%7Bb%28x%29%7D%7B%5Cfrac+%7B%5Cpartial+f+%7D%7B%5Cpartial+x%7D%7D%28x%2Ct%29%5C%2Cdt.&bg=ffffff&fg=333333&s=0&c=20201002