#k=1/[x+sqrt(x^2+y^2)]#
#color(white)("XXX")rarr x+sqrt(x^2+y^2)=1/k#
#color(white)("XXX")rarr sqrt(x^2+y^2)=1/k-x#
#color(white)("XXX")rarr x^2+y^2=1/k^2-2x/k+x^2#
#color(white)("XXX")rarr y^2=1/k^2-(2kx)/k^2#
#color(white)("XXX")rarr y=+-sqrt(1-2kx)/k#
Assuming #k# is a (non-zero) constant
and #y# is dependent on the variable #x#
#color(white)("XXX")y_x=+-sqrt(1-2kx)/k#