#d/dx(15) =d/dx(x^2+y^2)#
#0=2x+2y(dy)/dx#
#2y(dy)/dx=-2x# ..... See foot note!
#(dy)/dx=-x/y#
#color(blue)(~~~~~~~~~~~"Foot Note" ~~~~~~~~~~~~~~~)#
#color(brown)("Let "u=y^2)#
#color(brown)((du)/dy =2y)#
#color(brown)("So "(du)/dx = (du)/dy *(dy)/dx)#