https://s0.wp.com/latex.php?latex=%5Cdisplaystyle+%5Cfrac%7B%5Cpartial%7D%7B%5Cpartial+x_j%7D+%5Cint_%7B%28%7B%5Cbf+R%7D%5Ed%29%5Ep%7D%5C+d%5Cmu%28t%2Cx%29%5Ep%5C+dx+%3D+-+%5Cint_%7B%28%7B%5Cbf+R%7D%5Ed%29%5Ep%7D+%5B%5Cfrac%7Bx_j-y_j%7D%7B2t%7D%5D_%7B1+%5Crightarrow+p%7D%5C+d%5Cmu%28t%2Cx%29%5Ep%5C+dx.&bg=ffffff&fg=000000&s=0