https://s0.wp.com/latex.php?latex=%5Clim%5Climits_%7Bt%5Cto+0%7DD_1%28s%29%3D%5Cfrac%7B%5Cpartial+f%7D%7B%5Cpartial+X%7D&bg=FFFFFF&fg=111111&s=0&c=20201002