https://s0.wp.com/latex.php?latex=++%5Cdisplaystyle++%5Csum_%7Bk%3D1%7D%5E%7Bn-1%7D+%5Csum_%7Bj%3Dk%2B1%7D%5E%7Bn%7D+k+%5Ccdot+j++%3D+++%5Cfrac%7B1%7D%7B2%7D+%5Ccdot+++%5Cleft+%28++%5Csum_%7Bk%3D1%7D%5E%7Bn%7D++k%5E%7B3%7D++-++%5Csum_%7Bk%3D1%7D%5E%7Bn%7D++k%5E%7B2%7D++++%5Cright+%29++%3D+++%5Cfrac%7B1%7D%7B2%7D+%5Ccdot+++%5Cleft+%28++%5Csum_%7Bk%3D1%7D%5E%7Bn%7D++k%5E%7B2%7D++%5Cleft+%28+k-1+%5Cright+%29+++%5Cright+%29++&bg=ffffff&fg=000000&s=0