where the 2 went ? please view link, they multiplied by 2 on both sides and 2 dissapered.

please tell me how this thing works.

i mean does xdx + ydy = d(x² + y²) ?

then is the solved one wrong plez tell.

they dont let me add link so please copy and paste and remove the spaces.

https://www.dropbox.com /s/h75lljdnku7g9 8r/Screenshot 20 13-11-09 1 2.49.18.png